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Mastering Run-On Sentence Identification: The Essential SAT Writing Guide to Clause Tests and Fixes
Spot comma splices and fused sentences quickly using proven SAT grammar and punctuation strategies.
Run-on sentences are a staple of the SAT Writing and Language section, often disguised by sophisticated vocabulary or transition words. This guide breaks down the structural mechanics of independent clauses, providing you with a reliable three-step test to identify errors under time pressure. Learn exactly when to deploy a semicolon, period, or coordinating conjunction to ensure every sentence on your exam is grammatically perfect.
January 21, 2026
January 21, 2026
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Which of the following best describes a run‑on sentence?
These answers cover the main questions connected to this SAT topic, so you can review the key points quickly. They also give you a faster way to revisit the practical takeaways without rereading the full article.
A run-on occurs when two independent clauses—each capable of standing alone as a complete sentence—are joined without proper punctuation or a valid joining word. The two main types are fused sentences (no punctuation between clauses) and comma splices (a comma but no conjunction). On the SAT, recognizing when two complete thoughts are incorrectly fused is essential for scoring well on Writing and Language questions.
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Apply a three-step clause test: find all verbs and identify who or what performs each action, then determine if you have two complete thoughts, and finally check whether a valid connector (period, semicolon, or comma plus FANBOYS) links them. If two complete thoughts are touching without a proper link, you've found a run-on. Practice scanning for FANBOYS and verifying that commas precede them when joining independent clauses.
You can fix a run-on by using a period to end one sentence and start another, a semicolon to connect closely related ideas, a comma plus a coordinating conjunction from the FANBOYS list (for, and, nor, but, or, yet, so), or subordination to make one clause dependent. The best fix depends on the logical relationship between the ideas: use semicolons or periods for neutral connections, and use comma plus FANBOYS or subordination when cause-and-effect or contrast is involved.
Transition adverbs like however, therefore, and consequently are not coordinating conjunctions, so they cannot join two independent clauses with only a comma. This creates a comma splice. To correctly connect clauses with these words, use a semicolon before the transition adverb (IC; however, IC) or create subordination with a word like although. The SAT frequently tests this pattern to see if you distinguish between true conjunctions and transitional adverbs.
FANBOYS stands for for, and, nor, but, or, yet, so—the seven coordinating conjunctions that can join two independent clauses. When a FANBOYS word links two ICs, a comma must precede it. If you see a FANBOYS word without a comma before it when joining two complete sentences, that is an error. Memorizing FANBOYS helps you quickly spot the correct fix on comma splice questions.
Run-ons often hide inside long descriptive phrases, parenthetical interruptions, or after transitional adverbs. One common disguise is a pronoun restart: 'The engineers meeting daily in the new workspace, they reported faster progress' is a run-on because 'they' launches a second independent clause after the descriptive phrase. Another trap is assuming long, sophisticated-sounding sentences are correct—trust structure over sound, and always verify that complete thoughts have proper connectors.
A run-on contains two complete independent clauses (each with its own subject and verb), while a fragment is missing one of these elements and cannot stand alone. To distinguish them, label each chunk: if both have a subject-verb pair capable of forming a complete sentence, you need a legal connector. A fragment like 'Because the museum expanded its collection' lacks the main clause needed to complete the thought, whereas 'The museum expanded its collection; it needed more display space' correctly joins two ICs.
