ACT English Grammar Rules

Why This Matters

The ACT English section tests whether you understand how sentences work. Every question falls into one of two categories: conventions of standard English (grammar, punctuation, sentence structure) or production of writing (organization, style, concision). The grammar rules in this guide form the backbone of that first category, appearing in roughly 50-60% of all English questions.

The ACT tests these rules in predictable patterns. Subject-verb agreement questions almost always involve interrupting phrases between the subject and verb. Pronoun questions test ambiguous antecedents. Punctuation questions check whether you know the difference between essential and non-essential clauses. Learning to recognize these specific traps will help you move through the English section with confidence and speed.

---

Agreement Rules

These rules ensure that related parts of a sentence match in number, person, or tense. Agreement errors create logical inconsistencies, and the ACT tests them constantly.

Subject-Verb Agreement

• Singular subjects take singular verbs; plural subjects take plural verbs. This sounds simple, but the ACT hides subjects behind interrupting phrases to trick you.
• Ignore prepositional phrases between the subject and verb. "The box of chocolates is" not "are," because box is the subject, not chocolates.
• Indefinite pronouns like everyone, each, nobody, and anyone are always singular, even when they seem to refer to groups. "Each of the students has" not "have."

Pronoun-Antecedent Agreement

• Pronouns must match their antecedents in number. If the antecedent is singular, the pronoun must be singular too.
• Watch for ambiguous antecedents. When two nouns could be the referent, the ACT wants you to replace the pronoun with the specific noun. For example, "Tom told Jerry that he was late" is ambiguous because he could refer to either person.
• Compound antecedents joined by "and" require plural pronouns, while those joined by "or/nor" match the closer antecedent.

Verb Tense Consistency

• Maintain the same tense throughout related clauses unless there's a clear reason to shift (like describing events at different times).
• Context clues like dates, time words (yesterday, by next year), or surrounding verbs tell you which tense is correct. Always read the full paragraph before choosing.
• Perfect tenses (had walked, has eaten) show completed actions relative to another point in time. Had walked (past perfect) means the action finished before another past event. Has eaten (present perfect) means the action started in the past and connects to the present.

---

Sentence Structure Rules

These rules govern how sentences are built and connected. The ACT tests whether you can recognize complete thoughts, proper connections, and logical modifiers.

Sentence Fragments and Run-Ons

• Fragments lack a subject, verb, or complete thought. Dependent clauses standing alone are the most common fragment type on the ACT. For example, "Because she arrived early." has a subject and verb but isn't a complete thought due to because.
• Run-ons and comma splices occur when independent clauses are joined incorrectly. A run-on smashes two sentences together with no punctuation. A comma splice uses only a comma between them. Fix either with a period, semicolon, or comma + coordinating conjunction.
• FANBOYS (for, and, nor, but, or, yet, so) can join independent clauses, but only when preceded by a comma.

Modifier Placement

• Modifiers must sit next to what they modify. A descriptive phrase at the start of a sentence should describe the subject that immediately follows the comma.
• Dangling modifiers occur when the intended subject isn't actually in the sentence. "Walking to school, the rain started" is wrong because the rain wasn't walking. It should be "Walking to school, she got caught in the rain."
• Misplaced modifiers create unintentionally absurd meanings. Ask yourself: what is this phrase actually describing?

Parallel Structure

• Items in a list must share the same grammatical form. All gerunds, all infinitives, or all nouns. Not a mix. "She likes running, swimming, and biking" is parallel. "She likes running, to swim, and bikes" is not.
• Look for conjunctions (and, or, but) and comparison words (than, as) as signals that parallelism is being tested.
• Paired constructions like not only...but also and either...or require parallel elements on both sides.

---

Punctuation Rules

Punctuation marks are traffic signals for readers. The ACT tests whether you know exactly when each mark is required and when it's not.

Comma Usage

• Use commas after introductory elements, to separate items in a list, and before FANBOYS conjunctions joining independent clauses.
• Non-essential information (phrases you could delete without changing the core meaning) gets set off by commas on both sides. "My brother, who lives in Texas, is visiting" uses commas because the clause is extra info. "Students who study regularly perform better" has no commas because the clause is essential to identifying which students.
• Never use a comma alone to join two independent clauses. That's a comma splice, and it's always wrong on the ACT.

Semicolon and Colon Usage

• Semicolons connect two independent clauses that are closely related. Think of a semicolon as a "soft period." Both sides must be able to stand alone as complete sentences.
• Colons introduce explanations, lists, or elaborations. The clause before the colon must be a complete sentence, but what follows doesn't have to be.
• Semicolons in lists separate items only when those items already contain commas: cities like Paris, France; London, England; and Tokyo, Japan.

Apostrophe Usage

• Possessive nouns use apostrophes. Singular nouns add 's (the dog's bone), while plural nouns ending in s add only an apostrophe (the dogs' bones).
• Contractions use apostrophes to mark missing letters. Don't means do not; it's means it is.
• Its vs. it's is heavily tested. Its (no apostrophe) is possessive, while it's is a contraction for "it is." If you can substitute "it is" into the sentence and it still makes sense, use it's. Otherwise, use its.

---

Style and Clarity Rules

These rules focus on making writing clear, direct, and effective. The ACT rewards concise answers. When two choices are grammatically correct, the shorter one is usually right.

Concision and Redundancy

• Eliminate wordiness. Phrases like "due to the fact that" should become "because." "In order to" should become "to."
• Avoid redundancy by cutting words that repeat the same idea. "Past history," "unexpected surprise," and "advance planning" all contain a word that restates what the other word already means.
• Choose the shortest grammatically correct answer unless a longer option adds necessary meaning or clarity.

Active vs. Passive Voice

• Active voice puts the doer of the action first and is more direct: "The scientist conducted the experiment."
• Passive voice flips this order and often adds unnecessary words: "The experiment was conducted by the scientist."
• Prefer active voice unless the actor is unknown or the passive construction serves a specific purpose in context.

---

Word Choice Rules

These rules test whether you know how to use words correctly in context. Memorization helps, but reading the full sentence for meaning is essential.

Commonly Confused Words

• Affect/effect: Affect is usually a verb meaning "to influence." Effect is usually a noun meaning "a result." "The policy will affect the effect on students."
• Their/there/they're: Their is possessive, there refers to a place or is used as an expletive ("there are"), they're means "they are."
• Who/whom: Who is a subject (who did it?), whom is an object (to whom?). Substitute he/him to check: if him works, use whom.

Idioms and Prepositions

• Idiomatic expressions require specific prepositions. You're capable of, not "capable to." Something is different from, not "different than."
• No rule predicts idioms. You have to recognize correct usage through familiarity and reading.
• Common tested idioms include regarded as, distinguish between, attribute to, and comply with.

Comparative and Superlative Adjectives

• Comparatives (-er or more) compare two things: "She is taller than her brother."
• Superlatives (-est or most) compare three or more: "She is the tallest in her class."
• Irregular forms must be memorized: good/better/best, bad/worse/worst, little/less/least.
• Never double up. "More better" and "most tallest" are always wrong.

---

Conjunctions and Transitions

These words show relationships between ideas. The ACT tests whether you can identify the logical connection between clauses.

Coordinating and Subordinating Conjunctions

• Coordinating conjunctions (FANBOYS) connect equal elements. Use a comma before them when joining independent clauses.
• Subordinating conjunctions (although, because, since, when, if) create dependent clauses that cannot stand alone.
• Placement matters. A subordinating conjunction at the start of a sentence requires a comma after the dependent clause: "Because it rained, the game was canceled." When the independent clause comes first, you typically don't need a comma: "The game was canceled because it rained."

Transition Words

Transition questions are really logic questions. You need to identify the relationship between two ideas and pick the word that expresses it.

• Contrast: however, nevertheless, on the other hand, although
• Cause/effect: therefore, consequently, as a result, because
• Addition: furthermore, moreover, in addition, also
• Example: for instance, specifically, in particular

Read the sentences before and after the transition to determine the relationship. Don't just pick the transition that "sounds good."

---

Quick Reference Table

---

Self-Check Questions

1. What do subject-verb agreement and pronoun-antecedent agreement have in common, and how do you identify what needs to "agree" in each case?

2. You see a sentence that begins with a descriptive phrase followed by a comma. What should you immediately check, and what error is the ACT likely testing?

3. Compare semicolons and colons: both require a complete sentence before them, but how do you decide which one is correct?

4. A question offers four answer choices, and two are grammatically correct but different lengths. What's the ACT's general preference, and what's the exception?

5. How would you fix this sentence, and which two rules does it violate? "Running through the park, the sunset was beautiful, it made everyone stop and stare."