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SAT Comma Splices and Run-On Sentences

A comma splice is two complete sentences joined by a comma alone. A run-on joins them with no punctuation at all. Both are among the most frequently tested errors on the SAT, and both are fixed by the same short list of legal joins.

The rule

Two complete sentences can be joined only by a period, a semicolon, a comma plus a coordinating conjunction, or by making one of them dependent.

First learn to recognize the problem. Read each side of the punctuation mark on its own and ask whether it could be a sentence. In: the storm passed quickly, the roads stayed closed for days, both halves are complete, so a comma cannot hold them together. That is a comma splice, and it is an error no matter how short the two halves are.

There are four standard fixes and the SAT uses all of them. Use a period and start a new sentence. Use a semicolon. Use a comma plus a coordinating conjunction such as and, but, or so. Or make one of the halves dependent by adding a word like because, although, or while at its front.

That last fix changes meaning as well as grammar, which is why the choices often differ in the connecting word. Because the storm passed quickly, the roads stayed closed for days asserts a cause that may contradict the passage. Although the storm passed quickly, the roads stayed closed for days states a contrast. Pick the connector that matches the passage, not just the one that fixes the grammar.

Conjunctive adverbs are the classic trap. However, therefore, moreover, and nevertheless are not conjunctions and cannot join two sentences with only a comma. Writing: the data were clear, however the conclusion was disputed, is still a comma splice. It needs a semicolon or a period before however, with a comma after it.

One caution against overcorrecting. Not every comma between two verbs signals a splice. In: the storm passed quickly and left little damage, the second half has no subject of its own, so it is not a complete sentence and no stronger punctuation is needed. Always test both sides for a subject and a verb before declaring an error.

The trap

Treating however or therefore as a conjunction and joining two complete sentences with a comma before it.

Try it

Try one
Real question

The 1908 expedition across the plateau carried only what four mules could ______ a canvas tent, a theodolite, and six weeks of dried food.

Which choice completes the text so that it conforms to the conventions of Standard English?

Cold-chain rules govern the samples from the moment they leave the field, and a single warm hour can make a season of collecting worthless. To keep the samples from warming during the long drive back to the ______ the team packed each vial in a thermos of crushed ice.

To keep the samples from warming during the long drive back to the ______ the team packed each vial in a thermos of crushed ice.

  • A) lab which was located three hours away
  • B) lab, which was located three hours away
  • C) lab, which was located three hours away, ✓
  • D) lab; which was located three hours away

Why: The clause 'which was located three hours away' interrupts between 'the long drive back to the lab' and 'the team packed each vial', so it needs a comma at each end: 'lab, which was located three hours away,'. 'lab, which was located three hours away' opens the interruption and never closes it, so the phrase runs into the main clause. 'lab which was located three hours away' uses no commas at all, which makes the clause read as essential to identifying the lab. 'lab; which was located three hours away' uses a semicolon, which requires an independent clause after it, and a relative clause beginning with 'which' is not one.

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Questions students ask

What exactly is a comma splice?

It is two independent clauses joined by a comma with no conjunction. Each clause has its own subject and verb and could stand alone as a sentence, which means a comma is too weak to connect them.

Is a long sentence automatically a run-on?

No. Length has nothing to do with it. A run-on is defined by two complete sentences being fused without proper punctuation, and a very long sentence with correct structure is not an error.

Guides that cover this

More on sentence boundaries