A semicolon joins two complete sentences that are closely related, and it separates items in a list when those items already contain commas. That's the whole rule. Semicolons look intimidating because almost nobody uses them in text messages, but on ACT English they're one of the most predictable marks on the page, and they hand you a shortcut that can eliminate two answer choices at once.
Job one: joining two independent clauses
Each side of a semicolon has to be able to stand alone as a sentence. The greenhouse stayed warm all winter; the tomato plants kept producing into February. Both halves have a subject and a verb, and both could end with a period. Swap in a comma instead and you've created a comma splice, which is one of the fastest ways to lose an easy point.
The clauses should also feel connected. A semicolon quietly says these two thoughts belong together, so the ACT tends to use it where a period would feel abrupt.
Job two: separating messy list items
When the items in a series already have commas inside them, commas alone turn the list into soup. Semicolons act as the bigger dividers: The exchange students came from Osaka, Japan; Lyon, France; and Cusco, Peru. This use shows up less often than the first, but when you see a list with internal commas, semicolons are almost always the answer.
The period test, and the shortcut it gives you
Anywhere a semicolon is correct, a period would also be correct. That makes the two functionally interchangeable, and it hands you a genuinely useful elimination tool. If two answer choices are identical except that one uses a semicolon and the other uses a period, neither one can be the answer, because a multiple choice question can't have two correct options. Cross both off and spend your time on the remaining two.
The flip side is just as handy. If you can't replace the semicolon with a period without breaking the sentence, the semicolon is wrong. Because the ferry was delayed; we missed the opening ceremony. fails immediately, since Because the ferry was delayed. is not a sentence.
However, therefore, and the rest
Words like however, therefore, moreover, nevertheless, and consequently are conjunctive adverbs. They connect ideas in meaning, but they have no grammatical power to join clauses, which is exactly why the ACT likes them. The recipe called for buttermilk, however we only had yogurt. is still a splice. Write it as The recipe called for buttermilk; however, we only had yogurt. Semicolon before, comma after. Compare that with a real conjunction like but, which works with just a comma in front of it.
Correct versus incorrect, side by side
- Incorrect: Nina rewrote the introduction; which finally matched the conclusion. Correct: Nina rewrote the introduction, which finally matched the conclusion.
- Incorrect: The lake froze early this year, skaters showed up by Thanksgiving. Correct: The lake froze early this year; skaters showed up by Thanksgiving.
- Incorrect: We invited three neighbors; the Ruizes, the Chens, and the Okafors. Correct: We invited three neighbors: the Ruizes, the Chens, and the Okafors. An introduction to a simple list is a colon's job, not a semicolon's.
- Correct: He had trained for months; therefore, the hills felt manageable.
A two-minute self-check
Take any semicolon you see and do two things. First, read only what comes before it and ask whether that could end with a period. Then do the same for what comes after. If both pass, the semicolon is safe. If either fails, look for a comma, a colon, or a restructured sentence instead. For lists, ask a different question: do the items already contain commas? If not, you probably want plain commas.
Where to go from here
Semicolons are worth drilling in small, concentrated batches, since the rule is narrow enough to master in a single sitting. thirty-six has concept quizzes that group these questions together, which makes the period test feel automatic instead of effortful. After that, look at how colons work, since the two marks get confused constantly, and then review the whole family in the ACT punctuation rules guide. With 50 questions in 35 minutes on the English section, the seconds you save with a clean elimination rule genuinely add up.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.