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ACT English Grammar Rules You Need to Know

A small set of rules decides most ACT English questions. Learn these and you can answer by rule, not by what sounds right.

1. Joining two complete sentences

A complete sentence (an independent clause) has a subject and a verb and can stand alone. You can join two of them in three ways: a period, a semicolon, or a comma with a joining word (for, and, nor, but, or, yet, so). A comma alone is a comma splice and is always wrong.

  • Correct: The lab closed early. Students went home.
  • Correct: The lab closed early; students went home.
  • Correct: The lab closed early, so students went home.
  • Wrong: The lab closed early, students went home.

2. Semicolons and colons

A semicolon needs a complete sentence on both sides. A colon needs a complete sentence before it, and what follows explains or lists. Never put a colon right after a verb or a preposition: write “The kit includes a map and a compass,” not “The kit includes: a map and a compass.”

3. Commas for extra information

Information you could remove without changing the sentence's meaning is set off by a pair of commas. Information that is needed to identify the noun gets no commas. Never place a single comma between a subject and its verb.

  • Nonessential: My brother, who lives in Denver, is a nurse.
  • Essential: The student who finished first gets the prize.
  • Wrong: The tallest tree in the park, was planted in 1920.

4. Apostrophes and look-alike words

  • Its means belonging to it. It's means it is or it has.
  • Their shows ownership, there points to a place, and they're means they are.
  • Singular possessive: the student's notebook. Plural possessive: the students' notebooks.
  • Plain plurals never take an apostrophe: three notebooks, not three notebook's.

5. Subject-verb agreement

Find the subject and match the verb to it. Ignore phrases that sit between them: in “The box of old letters was found,” the subject is box, not letters.

Practice questions

  1. The committee reviewed the budget on Monday ___ it approved the plan on Tuesday.

    • A. Monday, it approved
    • B. Monday; it approved
    • C. Monday it approved
    • D. Monday: it approved
    Show answer and solution

    Answer: B. Both sides are complete sentences, so a semicolon works. A comma alone is a comma splice, no punctuation is a run-on, and a colon does not introduce an explanation here.

  2. The collection of rare coins ___ displayed in the museum's main hall.

    • A. are
    • B. were
    • C. is
    • D. have been
    Show answer and solution

    Answer: C. The subject is “collection,” which is singular. “Of rare coins” is a phrase between the subject and the verb, so “is” is correct.

  3. The garden ___ best flowers bloom in June is open to the public.

    • A. whose,
    • B. whose
    • C. who's
    • D. it's
    Show answer and solution

    Answer: B. “Whose” shows ownership. “Who's” means “who is,” which does not fit. The clause is essential to identify the garden, so no commas are used.

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