ACT English Grammar Rules

Why This Matters

The ACT English section isn't just testing whether you can spot a comma error—it's testing 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.

Here's the key insight: the ACT loves to test 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. Don't just memorize rules in isolation—learn to recognize the specific traps the test sets, and you'll move through the English section with confidence and speed.

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Agreement Rules

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

Subject-Verb Agreement

• Singular subjects take singular verbs; plural subjects take plural verbs—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
• Indefinite pronouns like everyone, each, nobody, and anyone are always singular, even when they seem to refer to groups

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
• 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, or surrounding verbs tell you which tense is correct—read the full paragraph
• Perfect tenses (had walked, has eaten) show completed actions relative to another point in time; use them to establish sequence

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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
• Run-ons and comma splices occur when independent clauses are joined incorrectly; fix them with a period, semicolon, or comma + coordinating conjunction
• FANBOYS (for, and, nor, but, or, yet, so) can join independent clauses only when preceded by a comma

Modifier Placement

• Modifiers must sit next to what they modify—a phrase at the start of a sentence should describe the subject that immediately follows
• Dangling modifiers occur when the intended subject isn't in the sentence at all; the ACT often tests this with introductory phrases
• 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
• 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

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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
• Never use a comma alone to join two independent clauses—that's a comma splice, and it's always wrong

Semicolon and Colon Usage

• Semicolons connect two independent clauses that are closely related; think of them as a "soft period"
• Colons introduce explanations, lists, or elaborations—but the clause before the colon must be a complete sentence
• 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, while plural nouns ending in s add only an apostrophe
• 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"

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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" and "unexpected surprise" are classic traps
• Choose the shortest grammatically correct answer unless a longer option adds necessary meaning or clarity

Active vs. Passive Voice

• Active voice puts the subject first and is more direct: "The scientist conducted the experiment"
• Passive voice obscures the actor 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

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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 (to influence), effect is usually a noun (a result); "The policy will affect the effect on students"
• Their/there/they're—their is possessive, there is a place, 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

Idioms and Prepositions

• Idiomatic expressions require specific prepositions—you're capable of, not "capable to"; different from, not "different than"
• No rule predicts idioms; you must 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

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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

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Quick Reference Table

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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."