A sentence fragment is an incomplete sentence missing a subject, a verb, or a complete thought, while a run-on sentence incorrectly joins two or more independent clauses without proper punctuation or conjunctions. Identifying and correcting these errors is essential for writing clear, grammatically complete prose.
Practice identifying fragments and run-ons in texts. Revise them by adding missing elements (for fragments) or breaking them into separate sentences or using correct punctuation (for run-ons).
A complete sentence requires three things: a subject (who or what), a verb (what it does or is), and a complete thought (the idea finishes). A sentence fragment fails on at least one of these counts. It might be a phrase with no verb ("Running through the park"), a dependent clause that waits for a main clause to complete it ("Because she left early"), or even a clause with both subject and verb that still doesn't constitute a full thought ("Which surprised everyone"). The test is not length — a fragment can be quite long — but completeness.
Run-on sentences are the inverse error: instead of too little, there is too much joined incorrectly. A comma splice joins two independent clauses with only a comma: "She left early, everyone was surprised." A fused sentence omits punctuation entirely: "She left early everyone was surprised." Both errors arise from the same misunderstanding — that a comma can do the work of a full stop or a conjunction. It cannot. A comma is a pause, not a division.
The correction strategies map directly onto the two error types. For fragments, the fix is addition: add the missing element (supply the subject, add a verb, or attach the fragment to an adjacent sentence it depends on). For run-ons, the fix is division or connection: break into two sentences with a period; join with a semicolon if the ideas are closely related; use a coordinating conjunction (for, and, nor, but, or, yet, so — the FANBOYS) after a comma; or subordinate one clause to the other with a subordinating conjunction, making one idea dependent.
A useful diagnostic for suspected run-ons: can you insert a period and a capital letter between the two halves without changing the meaning? If yes, you likely have a run-on. For suspected fragments: can the group of words stand alone and answer "who did what?" completely? If not, it is a fragment. These two questions catch the vast majority of errors you will encounter in revision.
The deeper skill this topic builds is sentence-level awareness — the ability to feel where a complete thought ends and another begins. This awareness pays forward into every sentence you write, because controlling sentence boundaries is the foundation of clear, reader-friendly prose. Writers who cannot reliably identify their own fragments and run-ons cannot reliably control their own sentences.