A run-on sentence has nothing to do with length — a sentence can be short and still be a run-on, or long and still be perfectly correct. What makes it a run-on is joining two complete sentences (two independent clauses, each of which could stand alone) without the punctuation or connecting word that's supposed to be there. Here's how to actually spot one in your own writing, and the four ways to fix it.
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A fused sentence jams two complete sentences together with no punctuation at all: "I finished the essay I still need to proofread it." Both halves — "I finished the essay" and "I still need to proofread it" — are complete sentences on their own, which is exactly the problem: nothing signals where one ends and the next begins.
A comma splice is the more common version, and the one that trips up more careful writers, because it looks like it should be fine: two complete sentences joined by just a comma, with no conjunction. "I finished the essay, I still need to proofread it." The comma alone isn't strong enough to join two independent clauses — it needs backup, either a conjunction or different punctuation entirely.
The quickest way to check your own sentence is to mentally split it in half at the spot you're unsure about and ask whether each piece is a complete sentence by itself — a subject and a verb, expressing a full thought. If both halves pass that test independently, and they're currently joined by nothing or just a comma, you've got a run-on.
This is different from a sentence that's just long. "I finished the essay after three all-nighters, two pots of coffee, and a last-minute rewrite of the conclusion" is a single independent clause with a lot of detail piled onto it — not a run-on, because there's only one complete sentence there, not two jammed together.
Split it into two sentences with a period: "I finished the essay. I still need to proofread it." The simplest fix, and often the clearest.
Add a comma plus a coordinating conjunction (and, but, or, nor, for, so, yet): "I finished the essay, but I still need to proofread it." This is what was missing from the comma splice version — the comma alone wasn't enough, the conjunction was doing the real work.
Use a semicolon if the two ideas are closely related enough that a full stop feels too abrupt: "I finished the essay; I still need to proofread it." No conjunction needed with a semicolon — just make sure both sides are still complete sentences on their own.
Subordinate one clause to the other if one idea is really the main point and the other is supporting context: "Although I finished the essay, I still need to proofread it." This changes the relationship between the two ideas slightly, so pick it when that shift actually fits what you mean.
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