"Run-on" gets used for any sentence that feels too long, but that's not what the term means grammatically. A run-on is a specific structural error — two independent clauses crammed together without the punctuation or connecting word they need — and a sentence can be short and still be one, or long and still be fine. Here's the actual definition, the related comma splice, and how to fix both.
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A fused sentence — the formal name for what most people call a run-on — is two independent clauses (two complete thoughts that could each stand alone as their own sentence) pushed together with no punctuation or connecting word at all between them, according to writing-center guidance from the Community College of Rhode Island and the College of Coastal Georgia. "I finished my essay I went to bed" is a run-on at nine words. A 40-word sentence built from correctly joined clauses and subordinating conjunctions isn't one at all.
A comma splice is the close cousin: the same two independent clauses, but joined with only a comma instead of nothing. "I finished my essay, I went to bed" is a comma splice — closer to correct than a fused sentence, but still not a complete fix, since a comma alone isn't strong enough to join two full sentences.
Split it into two sentences: add a period (or a semicolon, if the two ideas are closely related) where the clauses meet. "I finished my essay. I went to bed" or "I finished my essay; I went to bed" are both correct, and Purdue's Online Writing Lab specifically recommends the semicolon option when there's no connecting word linking the two thoughts.
Add a comma plus a coordinating conjunction: for, and, nor, but, or, yet, so (the acronym FANBOYS covers all seven). "I finished my essay, so I went to bed" fixes the same sentence while actually showing the relationship between the two clauses — which a bare period doesn't do.
Rewrite one clause so it's no longer independent: turning one half into a dependent clause ("After I finished my essay, I went to bed") folds both ideas into a single, correctly structured sentence instead of just splitting or stitching them.
Run-ons and comma splices are hard to catch by rereading because they usually still make sense out loud — your own internal pause where the sentence should've ended covers for the missing punctuation, so the error only shows up on the page, not in how it sounds in your head. That's the opposite of something like a misspelled word, which looks wrong on sight.
StudyPilot's grammar checker flags this under its sentence-structure check — specifically called out alongside fragments and awkward phrasing — with a one-click fix for each one it finds, rather than just a generic "grammar error" flag. It checks grammar, spelling, punctuation, and passive voice in the same pass, with no word limit on the free daily checks, so it's a reasonable place to run a full essay before counting on catching every fused sentence yourself.
Paste your essay into StudyPilot's grammar checker for a full sentence-structure scan — fused sentences, comma splices, and fragments flagged with a one-click fix, no word limit, free every day.
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