SAT® · Reading and Writing: Boundaries — Within Sentences
The Comma and the Dependent Clause
A dependent clause gets a comma after it when it leads off the sentence, and no comma before it when it trails the main clause. The rule tracks word order, not clause length or meaning.
A dependent clause can't stand alone as a sentence: 'Because the tide went out' is a fragment waiting for something to finish it. When you put that fragment first, the reader is left hanging until the main clause arrives, so the comma marks the exact spot where the dependent clause ends and the real sentence begins. Think of it as closing off a runway: the dependent clause builds up speed, the comma is where it lands.
Flip the order and the problem disappears. 'The wreck became visible when the tide went out' states the main point first, so there's nothing left dangling and nothing for a comma to close off. The test for speed: find the subject-verb pair that could stand alone as a full sentence. If a dependent clause sits in front of it, drop a comma right where that clause ends. If the dependent clause comes after, leave the sentence unmarked.
This sits next to the broader boundary-punctuation rules on the test, where semicolons, colons and dashes join independent clauses, and commas often get misused to do that same job. The dependent-clause comma is different: it never joins two independent clauses, it only marks where a sentence-starting dependent clause hands off to the main one. Spotting that difference fast is the whole game under time pressure.
Key terms
- dependent clause
- A clause with a subject and verb that cannot stand alone as a complete sentence, often starting with a word like because, when, or if.
- independent clause
- A clause with a subject and verb that can stand alone as a complete sentence.
- subordinating conjunction
- A word like because, when, or if that attaches a clause to a main clause and makes it dependent.
- fragment
- A dependent clause or incomplete thought punctuated as if it were a full sentence.
Watch it explained
- How it worksHow does putting the dependent half first build a runway that the comma has to close off?
- How it worksHow do you separate a dependent clause from an independent one fast enough to punctuate under time pressure?
- Worked exampleWalk me through punctuating 'When the tide went out the wreck became visible' and then flipping the halves.
- Going furtherWhat happens to that comma when the dependent clause is only two words long?
Check yourself
End of topic.