Many SAT grammar questions test where one sentence ends and another begins. If you can tell a complete sentence from a fragment, you can solve most comma, semicolon, colon and dash questions with one simple test.
In the Reading and Writing section, Standard English Conventions questions ask you to choose the punctuation or grammar that makes a sentence correct. A large share of these questions test boundaries — how clauses and phrases are joined. The good news is that these questions follow clear rules.
The key skill: is it a complete sentence?
A complete sentence (an independent clause) has a subject and a main verb and expresses a complete idea:
- "The museum opened in 1990." — complete.
- "Which opened in 1990." — not complete (a fragment).
- "Opening in 1990." — not complete.
Most boundary questions become easy once you check whether the words before and after the punctuation are complete sentences. Learn more in the rule card on complete sentences and fragments.
Rule 1: two complete sentences need strong punctuation
You can join two complete sentences with:
- a period (full stop),
- a semicolon,
- a comma plus a conjunction (and, but, or, so, yet, for, nor),
- a colon, if the second explains the first.
You cannot join them with a comma alone. That error is called a comma splice.
The data were incomplete, the team repeated the survey. ✘
The data were incomplete; the team repeated the survey. ✔
See the card on comma splices and run-ons.
Rule 2: "however" is not a conjunction
Words like however, therefore and as a result are transitions, not conjunctions. They cannot join two sentences with commas alone.
The trial failed, however, the second worked. ✘
The trial failed; however, the second worked. ✔
Rule 3: semicolons and colons need a complete sentence before them
- A semicolon works like a period: both sides must be complete sentences. See semicolons.
- A colon needs a complete sentence before it; after it can be a list, an explanation or a complete sentence. See colons.
The recipe needs three ingredients: flour, water and salt. ✔
The recipe needs: flour, water and salt. ✘
Rule 4: extra information needs matching punctuation
Information that can be removed without changing the main meaning is set off by a matching pair — two commas, two dashes or two parentheses.
The bridge—completed in 1932—is now a landmark. ✔
The bridge—completed in 1932, is now a landmark. ✘
If the information is essential (it tells you *which* person or thing), use no commas: "Students who study regularly score higher." See nonessential information and essential information.
Rule 5: no single comma between subject and verb
A single comma should never separate a subject from its verb, even when the subject is long:
The scientists who led the project, published their results. ✘
See no comma between essential parts.
A three-step method for any boundary question
- Find the blank and look at the words on both sides.
- Test each side: is it a complete sentence?
- Choose the punctuation that matches: two complete sentences need period / semicolon / comma + conjunction; extra information needs a matching pair; a list after a complete sentence can take a colon.
Practise next
- Work through all 12 boundary rules in the SAT grammar rule cards, each with 10 practice questions.
- Read Transitions and Rhetorical Synthesis.
This guide is general study advice written by AUSLEX, which is not affiliated with the test providers. Test formats, fees and score requirements can change, so always check the official website before you book or apply.