Comma splices and run-on sentences

A complete thought has a subject and a verb and could be a sentence by itself. “The weather was sunny” is one. “We decided to go to the beach” is another. When you join two of them with only a comma, the comma is doing a job it cannot do, and the result has a name: a comma splice.

Leave the comma out entirely and the fault is the same, though it is called a run-on. The two versions feel different to read — one hesitates, one rushes — but the cause is identical, so the fixes are identical too.

There are three reliable repairs, and choosing between them is a writing decision, not a rule. A full stop makes two short sentences, which is punchy and slightly formal. A semicolon keeps them tightly joined in one sentence, which suits two halves that explain each other. A comma plus a joining word such as and, but or so, keeps the flow and names the relationship — cause, contrast, result.

The test to apply is one question: can each half stand alone? If both can, you have a choice of three repairs. If one half cannot stand alone, the comma is actually correct, and this is the sentence students fix by mistake. “When the weather turned sunny, we went to the beach” needs its comma, because the first half cannot be a sentence by itself.