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The 14 ACT Grammar Rules You Must Know

Posted by Justin Berkman | Feb 4, 2016 8:30:00 AM

ACT English



 The ACT English section consists of questions that test your knowledge of grammar and writing style. If you learn and understand the grammatical rules that are 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 related example questions from real ACTs.


How to Use This Guide

The ACT English section tests you on a number of grammatical concepts, and many of these appear consistently on every test. In this guide, I’ll provide explanations and examples for the ACT grammar rules that tend to be tested most often. Keep in mind that there are also style and organization questions on the ACT that are unrelated to specific grammar rules.

This article is a great study tool if you’re 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 a 30, you should also study the rules that are only rarely tested on the ACT.

If you want more examples or explanation of the rules I discuss in this article, or if you want to learn about all of the grammar rules that are tested on ACT English, you should read all of our articles on ACT English grammar rules


The 14 Most Common ACT Grammar Rules

These are the rules that are the most important for you to learn and understand to conquer the ACT English section. I linked each rule to the article in which it's explained more thoroughly. Also, you can find related grammar rules that are less important but appear on the ACT explained in the articles.


#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 like “who,” “that,” “which,” or “where." The basic rule is that non-restrictive clauses should be surrounded by commas but restrictive clauses shouldn't 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” never should 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,” the meaning of the sentence would be substantially changed. 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 the clause without changing the meaning of the sentence, the clause shouldn’t 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 isn’t 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 after removing the non-restrictive clause:

My day was incredibly boring.


The 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. Because 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; therefore, it must be surrounded by commas. The original sentence is correct, and the answer is F. Answer choices G and J are 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 place 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:

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 should 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 ACT.


Actual ACT Example



The comma before the preposition “of” is wrong. Also, there shouldn’t be a comma after “rights.” The correct answer is C. On the ACT, if you’re unsure whether or not  there should be a comma, you should 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, the most common type of run-on sentence that appears on the ACT. This is an example of a comma splice:

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


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. You can put a conjunction after the comma:

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


Alternately, 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




The clauses before and after the comma are independent. They could stand alone as sentences; therefore, this sentence is a comma splice. Adding a conjunction after the comma corrects the comma splice, and the correct answer is B. Answer choice C doesn’t work because the word “so” doesn’t make sense in the context of this sentence.


#4: Use the Fewest Words Possible

On 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.


This is the corrected version of the sentence:

 Melissa enjoys shooting at the gun range.


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



 That looks like a fancy gun, but I don't know 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. Similarly, answer choices B and C are incorrect because what’s already written in the sentence implies that her collection grows “gradually” and “with the passing of time.” The correct answer is D.



Wordiness and redundancy are similar errors. If a word or phrase is redundant, it can be eliminated without altering the meaning of the sentence. Check out this sentence with a redundancy error:

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. The sentence can be be corrected by 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. The word “dangerous” can be eliminated without changing the meaning of the sentence. The correct answer is J.



Jumping out of an airplane can put your life in peril.



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

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


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 his kitchen that his mom bought on their vacation. 


The way the sentence is written makes it seem like George’s mom bought the kitchen on their vacation. Here is a corrected version of the sentence:

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


Now it’s clear that his mom bought the plate 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 then the noun the phrase is describing. Here’s an example of a dangling modifier:

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 like Jane’s shopping cart was walking through the grocery store. There are a couple of ways to fix the sentence. You can place the noun that is 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.


Or you can place the subject in the introductory phrase:

While Jane was walking through the 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, that doesn’t make much sense. From this sentence, we can infer that a fire dies down. Answer choice J is clear, concise, and it corrects the dangling modifier.


#6: Keep Verb Tenses Consistent

The basic verb consistency rule is that verbs should remain consistent in tense and form throughout a sentenceHere’s an example of a consistency error:

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.




Also, on the ACT, the verb tenses of surrounding sentences can provide context clues for the proper tense to use in a given sentence to maintain consistency. This is 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 are in the present tense. The shift to the past tense, “were,” doesn’t make sense given the context. Therefore, we need to 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 tend to 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

In case you don’t know, all of the words with apostrophes are contractions: it’s=it is, they’re=they are, who’s=who is, and would’ve=would have. Also, “its,” “their,” and “whose” show possession. “Would of” is always wrong and should be corrected to “would have” or “would’ve.” On the ACT, you may also see the word "its'," which is not a real word and will 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. We should use the possesive 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 involves synonyms or related words. On 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 the sentence indicates that the underlined word should signify that the celebration is made bigger or more important. Even though “heightened,” “raised,” and ‘lifted,” can mean making something higher, only “heighten” can mean to deepen or intensify. Therefore, “heightened” is the only word that works in this sentence, and the answer is A. The celebration isn’t literally lifted up, but it’s made more important.

These types of questions can be difficult because they require knowledge of vocabulary. They’re hard to prepare for because you don’t know which words will appear on the ACT. However, if you’re able to recognize a word choice question, hopefully you’ll be able to select the right word in the context of the given 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 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:

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 type on ACT English in 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 that are constructed by using the word “to” plus the 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 to rely on to know when to use a gerund or infinitive in an expression. 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.” The 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. This is the corrected version:

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. To form a possessive, if the word is singular or plural but doesn’t end in “s,” then you add an “s” after the apostrophe. 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 does end in “s,” just add an apostrophe after the “s.” Check it out:

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




Actual ACT Example



The sentence is referring to the age of one person. Therefore, the correct version of the possessive pronoun is “person’s.” The 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. If you end the sentence where the colon is placed, the sentence should make sense and be a complete thought. 

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 G.


#12: Semicolons Separate Two Complete Thoughts

Semicolons are like periods; they separate two independent clausesSemicolons should be able to be replaced with periods. On the ACT, you may find a randomly placed semicolon, or you may need to replace a comma with a semicolon to fix a comma splice. Here are example sentences with semicolons used 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



In this sentence, the semicolon is just randomly placed after the verb “are.” There isn’t an independent clause on either side of the semicolon and no punctuation is necessary after “are.” The correct answer is 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’s 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, can be used for people or things

For many situations, more than one relative pronoun can work. However, in other situations, only one is 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.



Breadsticks! (apasciuto/Flickr)


The relative pronoun is referring to "restaurants," a location, so “where” is more concise and correct. Check out 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 may have no idea when to use “who” vs. “whom,” but the rule isn’t overly complicated. Basically, “who” is used as a subject and “whom” is used as an “object.” A subject does an action or is being described and an object has something done to it. An object can be an object of a verb or preposition. Take a look at the article on pronoun case for more information about subject case vs. object case.

Here’s an example sentence using “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.

Here’s an example sentence using “whom”:

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


In this sentence, “whom” is 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 our correct answer. Answer choice D is wrong because the “they” after “that” is unnecessary. The correct answer is B. The relative pronoun comes before a verb, so you can’t use “whom.” 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 doesn’t in the plural form. In the example sentence, the subject is “rock stars,” which is plural; therefore, the verb should be in the plural form.

On the ACT, subject-verb agreement questions can be more difficult because there may be a phrase separating the subject from the verb. Check out this example:

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. The prepositional phrase “in my bedroom” separates the subject from the verb. If a verb is underlined on the ACT, make sure that you identify the subject that corresponds with that verb to ensure that the subject and verb agree. 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. From 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 are the most often tested on ACT English, there are some other grammar rules that will be tested. Here are links to other articles that explain the remaining grammatical rules you 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 like add and delete, author main goal, transitions, and macro logic.

Also, master the five critical concepts you must understand to ace ACT English.

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


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Justin Berkman
About the Author

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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