Question type
Run-on sentences and comma splices on the SAT
Two independent clauses can be joined in three correct ways: a period, a semicolon, or a comma with a coordinating conjunction (and, but, or, so, yet, for, nor). A comma alone is a comma splice; no punctuation at all is a run-on. Check whether the words after the blank could stand alone as a sentence.
Two complete sentences meet at the blank, and the choices offer different ways of joining or separating them.
College Board's question bank has 63 of these, about half rated Hard: the ones where the second clause starts with a pronoun or a participle and seems to continue the first.
- 63 in College Board's question bank, 51% rated Hard
- 25 practice questions of this type
- Reading it aloud misleads
Why students miss it
The comma splice: "pulsations, cooler outer layers absorb" sounds natural and joins two sentences with only a comma.
Semicolon before a conjunction: "; but" is not standard where ", but" joins the clauses.
A second clause that starts with "this", "it" or "today": it looks dependent but is a full sentence.
How to spot it
- A blank between two groups of words that each have a subject and a verb.
- Choices that differ between period, semicolon, comma and nothing.
The method
Two sentences or one?
Test both sides of the blank: could each stand alone as a sentence?
- Both can stand alone, no conjunction
- period or semicolon.
- Both can stand alone, with and/but/so
- comma before the conjunction.
- One side cannot stand alone
- a comma or nothing, depending on the structure.
Does reading it aloud help?
A comma splice reads aloud exactly like correct punctuation.
- Find the subject and verb on each side of the blank.
- Two complete sentences need a period, a semicolon, or a comma with a conjunction.
Two real questions
From the 1500prep bank, the way the app shows them. Answer first, then open the explanation.
In 1596 the Frisian pastor David Fabricius noticed a star in the constellation Cetus brightening unexpectedly for weeks, then fading until it vanished from view entirely. Later astronomers determined that this star, eventually named Mira, alternately expands and contracts over roughly eleven months in rhythmic outer layers periodically absorb much of its visible light.
Which choice completes the text so that it conforms to the conventions of Standard English?
Show the answer and explanationThe answer: pulsations. Cooler
Astronomers long assumed that most of the universe's gold formed inside exploding calculations in the 1990s showed that supernovae rarely generate enough neutrons to build such heavy atoms. In 2017, detectors recorded gravitational waves from two colliding neutron stars, and telescopes observed the light of newly synthesized elements spreading from the debris.
Which choice completes the text so that it conforms to the conventions of Standard English?
Show the answer and explanationThe answer: stars, but
Questions students ask
What is a comma splice?
Two complete sentences joined only by a comma. Fix it with a period, a semicolon, or a comma plus a conjunction such as and, but or so.
Can you use a semicolon before but?
Not in standard SAT usage. Two clauses joined by but take a comma before the conjunction.
Practise this type until it is automatic
Five more sentence-boundary questions, checked instantly, with every wrong choice explained. Free, no card.