Fused Sentences and Run-Ons

The lights went out everyone screamed looks like one sentence because it reads in one breath, but grammar only cares about complete thoughts. 'The lights went out' can stand alone. 'Everyone screamed' can stand alone. Two stand-alone sentences sitting next to each other with nothing holding them apart is a fused sentence. A comma splice is the same crime with a weak accomplice: someone drops a comma in the gap, as in 'The lights went out, everyone screamed,' which feels safer but still fails, because a comma alone can't join two complete sentences.

There are exactly four legal joints: a period or semicolon to split them into two sentences, a comma plus a coordinating conjunction like and or but, or a subordinating word that turns one clause dependent, like 'when the lights went out, everyone screamed.' Take 'The oven beeped the bread was done.' You can write it as 'The oven beeped. The bread was done,' or 'The oven beeped; the bread was done,' or 'The oven beeped, and the bread was done,' or 'When the oven beeped, the bread was done.' All four are correct; a fifth option, just a comma with no conjunction, is not.

Length never decides this. A sixty-word sentence built from one subject, one verb, and a chain of modifiers is fine; a six-word sentence with two complete thoughts crammed together is broken. This matters most when a dependent clause opens the sentence and hides a fused pair behind it, so the fix is to strip away the opening clause first and test what's left on its own. That stand-alone test is the same one used to catch fragments, just pointed in the opposite direction: a fragment fails for having too little, a run-on fails for having too much.