The 14 ACT Grammar Rules You Must Know


The ACT English section consists of questions that test your knowledge of grammar and writing style. By learning and understanding the grammar rules tested on the ACT, you'll be well on your way to getting an excellent ACT English score. In this article, I'll explain the most important ACT English grammar rules and provide sample questions from real ACTs.


How to Use This Guide to ACT Grammar Rules

The ACT English section tests numerous grammatical concepts, with many appearing consistently on every test. In this guide, I’ll give you explanations and examples for the most commonly tested ACT grammar rules. Keep in mind that there are also style and organization questions on ACT English that are unrelated to specific grammar rules.

This article is a great study tool if you are trying to get a middle score, just starting your ACT English studying, or reviewing the most important ACT grammar rules. If your target score is above 30, you should also study the grammar rules that are only rarely tested on the ACT.

For more examples or explanations of the rules I discuss below, or if you want to learn more about ACT grammar rules, read our other articles on ACT English.


The 14 Most Commonly Tested ACT Grammar Rules

These rules are by far the most important for you to learn and understand in order to conquer the ACT English section. I've linked each rule to the article in which it's explained more thoroughly. You can also find in these articles explanations for related grammar rules that are less important but still tested on the ACT.


#1: Surround Non-Restrictive Clauses and Appositives With Commas

Comma rules are extremely important to know for the ACT, and these comma rules are repeatedly tested on ACT English.


Relative Clauses: Restrictive vs Non-Restrictive

Relative clauses are dependent clauses that describe a noun and start with a relative pronoun or adverb such as "who," "that," "which," or "where." The basic rule is that non-restrictive clauses should be surrounded by commas, whereas restrictive clauses should not be.

On the ACT, clauses that start with "which" are always non-restrictive, and clauses that start with “that” are always restrictive. Therefore, clauses that start with “which” should always be surrounded by commas, while those that start with “that” should never be.


What's a Restrictive Clause?

Restrictive clauses are essential to the meaning of the sentence. You can’t take a restrictive clause out of a sentence without significantly changing the meaning of the sentence. Here’s an example:

People who wear sunglasses indoors aren’t invited to the party.

If you remove the clause "who wear sunglasses indoors," you substantially change the meaning of the sentence. You wouldn't know which people aren't invited, and you’d be left with "People aren’t invited to the party." Because you can’t remove this clause without changing the meaning of the sentence, the clause should not be surrounded by commas.



This kid can't come to the party. (Court Kizer/Flickr)


What's a Non-Restrictive Clause?

A non-restrictive clause is not essential to the meaning of the sentence. If you got rid of the clause, the sentence would still make sense, and its meaning wouldn’t change. Here’s an example sentence with the non-restrictive clause underlined:

My day, which consisted of eating and studying calculus, was incredibly boring.

The clause "which consisted of eating and studying calculus" adds more information about my day, but if it were removed, the overall meaning of the sentence would be unchanged.

This is what the sentence looks like if we remove the non-restrictive clause:

My day was incredibly boring.

As you can see, the overall meaning of the sentence is the same. The sentence is still describing my day as boring; we just have less information detailing why it was boring. 


Actual ACT Example



The clause "who had gathered essential material critical to writing his best-selling novel Roots from a griot in Gambia" is a non-restrictive clause which adds more information about Alex Haley. Since a non-restrictive clause must be surrounded by commas, the correct answer is D.


What Is an Appositive?

An appositive is a descriptive phrase that doesn’t include a verb. Like a non-restrictive clause, an appositive can be removed without changing the meaning of the sentence. Here’s an example sentence with the appositive underlined:

Lily, my niece, is the most exceptional child in the world.

If we get rid of the appositive, the sentence still has the same meaning:

Lily is the most exceptional child in the world. 



This isn't my niece.


 Actual ACT Example


The phrase "known as the Navajo code talkers" is an appositive and therefore must be surrounded by commas. Because the original sentence is correct, the correct answer is F. Answer choices G and J are both wrong because there is no comma after "group." Also, answer choice H is incorrect because it creates a run-on sentence.


#2: Don't Put a Comma Before or After a Preposition

On the ACT, it’s always incorrect to put a comma after a preposition and very rarely correct to place one before. Here are some example sentences that include commas incorrectly placed before or after prepositions:

 Ana enjoys traveling, to Hawaii for fun.

 Lucy was petrified to look under, the bed.

The commas before "to" and after "under" should be removed. Here are the corrected versions of the sentences:

Ana enjoys traveling to Hawaii for fun.

Lucy was petrified to look under the bed.





The one exception to this rule is when a preposition introduces a non-restrictive clause. Here’s an example of this:

Cade, with whom I went to college, is an extremely skilled physician.

The clause "with whom I went to college" is a non-restrictive clause that provides more information about Cade. Because non-restrictive clauses must be surrounded by commas, the comma before the preposition "with" is correct.

Keep in mind that this situation rarely comes up on the ACT. Generally, commas shouldn’t be put before or after a preposition on the test.


Actual ACT Example



The comma before the preposition "of" is wrong; there also shouldn’t be any comma after "rights." Therefore, the correct answer is C. On the ACT, if you’re unsure whether or not there should be a comma, it's best to err on the side of leaving the comma out.


#3: Don't Separate Two Independent Clauses With a Comma

Separating two complete thoughts with a comma is a grammar error known as a comma splice, and it's the most common type of run-on sentence that appears on the ACT. Here's an example of a comma splice:

I’m going to my friend’s house, it’s really far away.

As you can see, the clauses before and after the comma are complete thoughts that could stand alone as sentences.

There are a few ways to correct a comma splice. One is to place a conjunction after the comma:

I’m going to my friend’s house, but it’s really far away.

Alternatively, you can put a relative pronoun after the comma:

 I’m going to my friend’s house, which is really far away.

Finally, you can use a semicolon to correctly separate two complete thoughts:

 I’m going to my friend’s house; it’s really far away.



I'm not sure what this means. (Laura Olin/Flickr)


Actual ACT Example




Both clauses before and after the comma are independent and could stand alone as sentences; therefore, this sentence is a comma splice. Since adding a conjunction after the comma corrects the comma splice, the correct answer is B. While answer choice C also adds a conjunction, this choice doesn’t work since the word "so" doesn’t make sense in the context of the sentence. ("So" implies a cause-effect relationship, whereas "and" connects two related thoughts.)


#4: Use the Fewest Words Possible

When it comes to the ACT, the shortest, grammatically correct answer choice that expresses the same information as the original sentence will be the right answer. Sentences that are more concise are easier to comprehend.



On ACT English, wordiness is a grammatical error in which words or phrases are added to a sentence unnecessarily. Here's an example of a wordy sentence:

Melissa enjoys having fun by way of shooting at the gun range.

And here is the corrected version of the sentence:

Melissa enjoys shooting at the gun range.

Clearly, the second sentence is more concise, and it still contains all the relevant information that’s in the first sentence.



 That looks like a fancy gun—but, then again, I don't know anything about guns.


Actual ACT Example



The phrase "as time goes by" is unnecessary—it doesn’t add any information that can’t be inferred without it. Answer choices B and C are incorrect because the sentence already implies that her collection grows "gradually" and "with the passing of time." Thus, the correct answer is D.



Wordiness and redundancy are similar errors. If a word or phrase is redundant, this means it can be eliminated without altering the meaning of the sentence. Here is an example of a sentence with a redundancy error (the underlined part):

I quickly finished the test in a rapid manner.

The phrase "in a rapid manner" is redundant because the word "quickly" already implies that I finished the test rapidly. This sentence can therefore be corrected by simply getting rid of the redundant phrase:

I quickly finished the test.


Actual ACT Example



The word "peril" means danger; therefore, the phrase "dangerous peril" is redundant—that is, the word "dangerous" can be eliminated without changing the meaning of the sentence. This means the correct answer is J.



Jumping out of an airplane can put your life in peril. (Morgan Sherwood/Flickr)


#5: Modifiers Must Be Next to What They're Modifying

The general rule regarding modifiers is that they must be placed next to whatever it is they're modifying. On ACT English, there are two types of modifier errors, which we introduce below.


Misplaced Modifiers

Generally, adjectives and adverbs go before the word they’re modifying, and prepositional phrases go after the word they’re modifying. Here's an example of a misplaced modifier:

George broke the plate in the kitchen that his mom bought on their vacation. 

The way the sentence is written makes it seem as though George’s mom bought the kitchen (instead of the plate) on their vacation. Here is a corrected version of the sentence:

In the kitchen, George broke the plate that his mom bought on their vacation.

Now, it’s evident that George's mom bought the plate—not a kitchen!—on their vacation.


Actual ACT Example



The phrase "in pink-tinted glasses" should come after the word it’s modifying. From the sentence, we can safely assume that it was the woman who was in pink-tinted glasses. Answer choice G is the most logical, grammatically correct answer.


Dangling Modifiers

When a sentence begins with a modifying phrase, the introductory phrase must be immediately followed by a comma and the noun being described. Here’s an example of a dangling modifier, with the incorrect part underlined:

While walking through the grocery store, Jane’s shopping cart knocked three bags of Doritos onto the floor.

The way the sentence is written makes it seem as though Jane’s shopping cart—instead of Janewas walking through the grocery store. There are a couple of ways to fix the sentence. One is to place the noun that's being modified right after the comma:

While walking through the grocery story, Jane knocked three bags of Doritos onto the floor with her shopping cart.

Alternatively, you can place the subject in the introductory phrase:

While Jane was walking through the grocery story, her shopping cart knocked three bags of Doritos onto the floor.



Be careful with the Doritos. (theimpulsivebuy/Flickr)


Actual ACT Example




The original sentence implies that "she" died down; however, this doesn’t make much sense. From this sentence, we can infer that a fire dies down—not "she." Answer choice J is clear and concise, and it corrects the dangling modifier.


#6: Keep Verb Tenses Consistent

The basic verb consistency rule is that verbs should remain consistent in both tense and form throughout a sentence. Here’s an example of a consistency error, with the incorrect part underlined:

Twenty-five years ago, Josh bought Cross Colours clothing and learns how to rap.

The verb "learns" should be in the past tense. Not only should "learns" be consistent with the past tense "bought," but also the phrase "twenty-five years ago" indicates that this was something that happened in the past.

Here's the corrected version of the sentence:

Twenty-five years ago, Josh bought Cross Colours clothing and learned how to rap.




Furthermore, the verb tenses of surrounding sentences can provide context clues for the proper tense to use in a given sentence to maintain consistency.

Here's another example of a consistency error: 

Natalie works in fashion. She liked keeping up with the latest style trends.

The shift from the present tense "works" to the past tense "liked" doesn’t make sense in context, and the verb tenses should remain consistent. Here's a corrected version of the sentence:

Natalie works in fashion. She likes keeping up with the latest style trends.


 Actual ACT Example


The verbs "continue" in the first sentence and "score" in the second sentence are in the present tense; as a result, the shift to the past tense with the word "were" doesn’t make sense given the context. To fix this error, we must change "were" to the present tense. The correct answer is C. (Answer choice D unnecessarily adds the infinitive "to be.")


#7: Choose the Right Word Based on Context

Word choice is a common topic on the ACT English section. Word choice refers to knowing which specific word to use in the context of a given sentence. There are two common types of word choice questions that often appear on the ACT.



Homophones are words that sound the same but have different meanings. Here are some homophones that have appeared on the ACT:

  • its — it's
  • their — they're
  • whose — who's
  • would've — would of

All the words with apostrophes are contractions (it's = it is, they're = they are, who's = who is, and would've = would have). By contrast, "its," "their," and "whose" show possession. Finally, "would of" is always wrong and should be corrected to either "would have" or "would've." You might also come across the word "its'," which is not a real word and will thus always be incorrect.

Here is an example of a word choice error:

The committee chose not to defend it’s decision.

In this sentence, the use of the contraction "it's" is incorrect. Instead, we should use the possessive word "its" to indicate that the decision belongs to the committee: 

The committee chose not to defend its decision.


Related Words

The second type of word choice error on the ACT involves synonyms or related words. With this type of question, a word will be underlined and the answer choices will be related words. However, only one word will be correct given the context of the sentence.


Actual ACT Example



The context of this sentence indicates that the underlined word should signify that the celebration is made bigger or more important. Even though "heightened," "raised," and "lifted" can all mean making something higher, only the word "heighten" can mean to deepen or intensify. Therefore, the correct answer is A. The celebration isn’t literally lifted up but rather made more important.

These types of questions can be difficult because they require a deep knowledge of vocabulary. They’re hard to prepare for since you don’t know which words will appear on the ACT.

However, if you are able to recognize a word choice question, hopefully you’ll be able to select the right word using the context of the sentence. Also, you can keep track of word choice questions from previous ACTs.


#8: Use the Correct Idiomatic Expression

Idiom questions don’t conform to specific rules. You have to rely on your intuitive grasp of English and your knowledge of specific phrases to choose the right idiomatic expression. On most idiom questions on the ACT, you’ll be asked either to determine which preposition to use in a given sentence, or whether to use a gerund or an infinitive.


Prepositional Idioms

For prepositional idioms, you need to know which preposition to use based on the context of the sentence. Here’s an example sentence with a prepositional idiom error underlined:

Because Alexis was three hours late, I was worried of her. 

The correct expression is "worried about." There's no specific rule you can learn to identify this error; you just have to be familiar with the phrase. Idiom questions are the only types on ACT English for which it's in your best interest to rely on what sounds right.


Idioms With Gerunds or Infinitives

Gerunds are verbs that act as nouns and end in "ing." Examples of gerunds include "running," "talking," and "singing." Infinitives are verbs used as nouns; they are constructed by putting the word "to" before a verb. Examples of infinitives are "to run," "to talk," and "to sing."

While gerunds and infinitives can be interchangeable in some sentences, other sentences require the use of one or the other. Here’s an example of an idiom error:

You decided reading this article. 

In the sentence, the gerund "reading" should be changed to an infinitive:

You decided to read this article.

Unfortunately, there is no specific rule you can rely on to know when to use a gerund or infinitive in an expression. Try to use your knowledge of idioms and keep track of idiom questions on previous ACTs.


Actual ACT Example



The correct expression is "right to wear clothing," so the correct answer is C.





#9: A Pronoun Must Agree With Its Antecedent

This rule means that a pronoun must agree with its antecedent in number. A plural pronoun must refer to a plural noun, and a singular pronoun must refer to a singular nounHere's an example of a pronoun number agreement error:

Marshall forgot their homework. 

The pronoun "their" is referring to the homework of Marshall. Because Marshall is one person and "their" is a plural pronoun, this sentence has a pronoun agreement error.

Here is the corrected version of this sentence:

Marshall forgot his homework.


Real ACT Example



In this sentence, the plural pronoun "them" refers to the Navajo language, which is singular. The pronoun "it" at the end of the sentence also indicates that the underlined pronoun should be in the singular form. The correct answer is C. Answer choice B is wrong because it has a subject-verb agreement error.


#10: Use Apostrophes Correctly to Form Possessives

There are almost always a couple of apostrophe questions on the ACT. If you know apostrophe rules, these questions should be relatively simple.

If a word is singular or plural and does not end in "s," you add an "s" after the apostrophe to make it possessive. Here are a couple of examples:

I am the people’s champion.

Joe’s career isn’t going very well. 

To create a possessive for a plural word that ends in "s," just add an apostrophe after the "s." Here is an example of this:

The traditionalist thought that all of the basketball players’ shorts were too long.




Actual ACT Example



This sentence is referring to the age of one person; therefore, the correct version of the possessive pronoun is "person’s." The correct answer is A. Answer choice D is wrong because a comma can’t come between a subject and a verb.


#11: Colons Must Come After a Complete Sentence

Colons are usually used to introduce lists or explanations. The key rule for colons is that they must come after a complete sentence. So if you were to end the sentence where the colon is placed, the sentence should make sense and be a complete thought. Here is an example:

Incorrect: Sabrina needs to purchase a few items for her project like: construction paper, paint, and glitter.

Correct: Sabrina needs to purchase a few items for her project: construction paper, paint, and glitter. 

The first sentence is incorrect because the part of the sentence that comes before the colon isn’t a complete thought.


Actual ACT Example



In this sentence, the colon is unnecessary and improperly placed. The part before the colon isn’t a complete thought. In answer choices H and J, the commas are unnecessary. The correct answer is therefore answer choice G.


#12: Semicolons Separate Two Complete Thoughts

Semicolons are like periods; they separate two independent clauses. As a result, you should be able to replace any semicolon with a period.

On ACT English, you might find a randomly placed semicolon or need to replace a comma with a semicolon in order to fix a comma splice. Here are two example sentences with semicolons used both correctly and incorrectly:

Incorrect: Because Dave wanted an adrenaline rush; he decided to go skydiving.

Correct: Dave wanted an adrenaline rush; he decided to go skydiving.

The second sentence is correct because the clauses on both sides of the semicolon are independent and could stand alone as sentences.


Actual ACT Example



Here, the semicolon is just randomly placed after the verb "are"; there is no independent clause on either side of the semicolon. Since no punctuation is necessary after "are," the correct answer is answer choice C.


#13: Use the Correct Relative Pronoun 

The ACT likes to test your knowledge of relative pronouns. There are two specific relative pronoun rules that are important to know for ACT English.


Relative Pronouns Must Agree With Their Antecedents

Here are what different relative pronouns can be used to refer to:
  • who and whom — people only
  • when — specific times or time periods only
  • where — places only
  • which — any noun other than a person
  • that — any noun
  • whose — possessive that can be used for people or things

In many situations, more than one relative pronoun can work. However, in other situations, only one will be acceptable. Here's an example of a relative pronoun error:

Incorrect: I love going to restaurants in which I can get unlimited breadsticks.

Correct: I love going to restaurants where I can get unlimited breadsticks.

The relative pronoun here is referring to "restaurants"—a location—so "where" is more concise and appropriate.



Breadsticks! (apasciuto/Flickr)


Here is another example:

Incorrect: Johnny enjoys books where he gets to choose his own adventure.

Correct: Johnny enjoys books in which he gets to choose his own adventure.

A book isn’t a location; therefore, "where" is the incorrect relative pronoun. Media, including books, movies, and articles, should be modified with "in which."


Who vs Whom

You might have no idea when to use "who" or "whom," but the rule isn’t actually that complicated. Basically, "who" is used as a subject and "whom" is used as an object. A subject does an action or is being described, whereas an object has something done to it. An object can be an object of a verb or preposition. Take a look at our article on pronoun case for more information about subject case vs object case.

Now, let's take a look at a couple of examples. First, here is an example sentence using the relative pronoun "who":

James, who is my friend, lives in Oklahoma City. 

In this sentence, "who" is being used correctly. The word "who" is modifying James, and he is my friend.

Next, here's an example sentence that uses the relative pronoun "whom":

My aunt, from whom I got this jacket, is a very interesting woman.

In this sentence, "whom" is also being used correctly. The word "whom" modifies my aunt; I got the jacket from her. 

You should be able to substitute he/she/they for "who" and him/her/them for "whom." Also, always use "who" before a verb and "whom" after a preposition.


Actual ACT Example



Remember that "where" can only be used to modify a location. Because storytellers are people, "where" is incorrect. Now, let’s find the right answer. Answer choice D is wrong because the "they" after "that" is unnecessary. Additionally, since the relative pronoun comes before a verb, you can’t use "whom."

This means that the correct answer is B. The word "who" modifies the storytellers who are doing the action. 


#14: Subjects and Verbs Must Agree

Subjects and verbs must agree, meaning that you must use the singular form of a verb with a singular subject and the plural form of a verb with a plural subject. Take a look at these example sentences:

Incorrect: Rock stars likes to entertain adoring fans.

Correct: Rock stars like to entertain adoring fans.



The King!


If the verb is in the present tense and the subject is in the third person (he/she/it/they), the verb usually ends in "s" in the singular form and does not in the plural form. In the example above, the subject is "rock stars," which is plural; therefore, the verb "like" should also be in the plural form ("like" instead of "likes").

On the ACT, subject-verb agreement questions can be even more difficult if there is a phrase that separates the subject from the verb. Here's an example of this:

Incorrect: The clothes in my bedroom is in my closet.

Correct: The clothes in my bedroom are in my closet. 

The subject of the sentence is "clothes," which is plural, so the verb should be in the plural form. However, the prepositional phrase "in my bedroom" separates the subject from the verb.

If a verb is underlined, make sure you identify the subject that corresponds with the verb to ensure that the subject and verb agree with each other. Also, keep in mind that a subject can never be contained in a prepositional phrase.


Actual ACT Example


This is a relatively basic subject-verb agreement question. Looking at the previous sentence, we know that the verb should be in the present tense, so G and H are wrong. The subject of this sentence is "molecules," which is plural. Therefore, we should use the plural form of the verb "bump." The correct answer is J.


Other Grammar Rules on ACT English

While the rules I explained above are the most often tested on ACT English, there are some other grammar rules that will be tested as well. Here are the links to our other articles that explain the remaining grammatical rules you'll need to know for the ACT:


What's Next?

Now that you're comfortable with the grammar rules on ACT English, make sure you know about style and organization questions. These include questions dealing with add and delete options, author's main goals, transitions, and macro logic.

In addition, we recommend mastering the five critical concepts you must understand to be able to ace ACT English.

Finally, to excel on the ACT English section, you'll need to determine the best approach for reading passages.



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About the Author
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Justin Berkman

Justin has extensive experience teaching SAT prep and guiding high school students through the college admissions and selection process. He is firmly committed to improving equity in education and helping students to reach their educational goals. Justin received an athletic scholarship for gymnastics at Stanford University and graduated with a BA in American Studies.

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