Boundaries questions test punctuation between and within sentences: periods, commas, semicolons, colons, and dashes. The digital SAT is ruthless about a few specific rules — master them and these become free points.
Core test: Is each side of the punctuation an independent clause (could stand alone as a sentence) or not? That single check drives most answers.
The rules that show up constantly
- Two independent clauses need: a period, a semicolon, or a comma + FANBOYS conjunction (for, and, nor, but, or, yet, so). Never a comma alone (that's a comma splice).
- Colon — must follow a complete sentence; introduces a list, explanation, or example.
- Semicolon = usually works like a period: both sides must be complete sentences. One exception the SAT does test — in a list whose items already contain commas, semicolons separate the items ("Lima, Peru; Quito, Ecuador; and Bogotá, Colombia").
- No single, unpaired mark between a subject and its verb, and no commas around an essential (restrictive) phrase. A nonessential element may sit between subject and verb when it is enclosed by a matching pair of commas, dashes, or parentheses.
Fast trick: If the choices differ only in punctuation, mentally replace each with a period. If a period works, a semicolon works too — and a lone comma never will.
Worked examples
The lecture ran long ____ several students left early.
- Check each side: 'The lecture ran long' is a complete sentence. 'several students left early' is also a complete sentence.
- Two complete sentences cannot be joined by a comma alone (that's a comma splice).
- Correct options: a period, a semicolon, or a comma + FANBOYS ('long, so several…').
- So 'long; several students left early' is right.
The kit contained three tools ____ a hammer, pliers, and a wrench.
- Is the part before the blank a complete sentence? 'The kit contained three tools' — yes.
- What follows is a list that explains 'three tools.'
- A colon follows a complete sentence to introduce a list or explanation.
- So: 'three tools: a hammer, pliers, and a wrench.'
Common mistakes to avoid
- Using a comma to join two complete sentences. If both sides could stand alone, a comma alone is always wrong.
- Putting a colon after an incomplete sentence. 'The kit contained: a hammer…' is wrong — the part before a colon must be complete.
- Adding a comma between the subject and its verb. 'The tall boy in the blue coat, walked away' is incorrect.
- Using only one comma around nonessential information that interrupts a sentence. A nonessential phrase in the MIDDLE needs a comma on both sides; one at the very start or very end of the sentence takes just one.