A sentence fragment lacks a subject, a complete verb, or expresses an incomplete thought; it is a word group punctuated as a sentence but not grammatically complete. A run-on sentence fuses two or more independent clauses without proper punctuation or conjunction, including the comma splice (joining with only a comma). Identifying and correcting these errors is fundamental to clear, grammatically standard writing.
Present matched pairs of a fragment/run-on and its corrected version, asking learners to diagnose what is missing or misjoined before seeing the fix. Have learners scan their own drafts specifically for these error types.
You already know that an independent clause is a complete thought with a subject and a finite verb that can stand alone. A sentence fragment is anything punctuated as a sentence that fails this test. It may be missing a subject ("Running through the park." — who ran?), missing a complete verb ("The dog in the yard." — what happened?), or it may be a dependent clause written alone ("Although he tried." — tried to do what?). The dependent clause is the trickiest: it has a subject and a verb, but the subordinating conjunction signals incompleteness. Spot the subordinator, and you've spotted a likely fragment.
The failure mode of the fragment is incompleteness. The failure mode of the run-on sentence is the opposite: too much joined without the punctuation to hold it together. A run-on fuses two or more independent clauses as if they were one sentence. "She studied hard she passed the exam" — both are complete independent clauses with no separator between them. The comma splice is the most common specific form: "She studied hard, she passed the exam." A comma signals a pause, not a full join. It is not strong enough punctuation to hold two independent clauses together by itself.
From your study of compound sentences, you know the three legitimate ways to join independent clauses: a coordinating conjunction (FANBOYS: for, and, nor, but, or, yet, so), a semicolon alone, or a semicolon followed by a transitional adverb (however, therefore, therefore). Run-ons break these rules by omitting the required connector. The diagnostic question: can you identify two complete independent clauses in one "sentence" without a coordinating conjunction or semicolon between them? If yes, it's a run-on.
The most important clarification: these errors are about structure, not length. A long sentence with multiple properly connected clauses is not a run-on — it is a well-built complex or compound sentence. A three-word group missing a verb is a fragment regardless of its brevity. And fragments are not always wrong — skilled writers use them deliberately for effect: "Not a chance." But the intentional fragment is a deliberate choice by someone who could write the complete version and chose not to. The goal is always conscious control: knowing when you have a fragment or run-on, and either correcting it or choosing it purposefully.