Two sentences arrive at the same workshop. One is a hundred and four words long and never stops. The other is eleven words long and never starts. Both are marked "sentence error" in red, and both are fixed by the same three moves.
The names matter less than the diagnosis. A run-on is two or more complete sentences glued together without the right join. A fragment is a group of words that gestures at a sentence without being one. Neither is a moral failure; both are what happens when a writer is thinking faster than they are typing.
How to hear a run-on
Read the sentence out loud. A run-on reveals itself as a sentence that keeps going after it has already finished a thought.
The report was late the client was unhappy.
Two complete sentences, no punctuation between them. This is the plainest form, called a fused sentence, and it is surprisingly common in first drafts and nearly extinct in published prose, because it is the easiest error to hear.
The slightly more respectable form is the comma splice.
The report was late, the client was unhappy.
The comma suggests a relationship without specifying one. Readers tolerate it in informal writing and editors remove it everywhere else. The comma rules guide covers the cases in detail; the short version is that a comma alone cannot join two sentences.
The three repairs
Every run-on has exactly three fixes. Choose by meaning, not by habit.
A period. Two sentences become two sentences. This is the safest repair and the most common.
The report was late. The client was unhappy.
A semicolon. The two sentences stay together and the reader is told they are related.
The report was late; the client was unhappy.
A conjunction with a comma. The relationship gets named: "and," "but," "so," "because," "although."
The report was late, so the client was unhappy.
The three repairs are not interchangeable in tone. A period separates. A semicolon equates. A conjunction argues. If you want the reader to feel the cause, use "so." If you want the reader to hold both facts at once, use a semicolon. If the facts are simply adjacent, use a period.
Recognising a fragment
A fragment looks like a sentence and is missing one of three things: a subject, a verb, or a complete thought.
Walking to the station in the rain.
No subject. Nobody is doing the walking.
Because the deadline moved.
A subordinating conjunction ("because," "although," "when," "if," "since") turns a sentence into a dependent clause. On its own it waits for something that never comes.
The most common fragment in professional writing is the afterthought:
The proposal was rejected. Which surprised no one.
"Which surprised no one" has a subject and a verb, so it looks complete. It is a relative clause hanging off a sentence that has already ended, and standard edited prose attaches it to the sentence it belongs to.
Repairing fragments
Use the same three moves in reverse: attach, complete, or promote.
Attach it to the sentence it depends on.
Because the deadline moved, we shipped on Friday.
Complete it.
We walked to the station in the rain.
Promote it to a full sentence.
The proposal was rejected. That surprised no one.
Promoting is the repair writers forget. A fragment can often become a legitimate short sentence if it gets a subject and a finite verb. "That surprised no one" is complete and punchy. The fragment was never the problem; the missing subject was.
Why fragments survive in good writing
Advertising copy runs on fragments. So does fiction. "Not anymore." "Every time." "A mistake."
These work because the reader supplies the missing context instantly, and the brevity creates emphasis. In narrative prose a fragment is a rhythm choice. In a contract, a lab report, or an academic essay it reads as an error, because the register promises completeness.
The test is whether the fragment can stand alone without the reader guessing. "Because the deadline moved" cannot. "A mistake." can.
The long sentence that is neither
Some sentences are long, complex, and completely correct. Length alone does not make a run-on.
Although the report arrived three days late, the client accepted the delay without complaint, and the project closed on budget.
Four clauses, one sentence, no error. The clauses are joined by a subordinator, a comma with a conjunction, and a comma with "and." Each join is legal. The sentence length and rhythm guide explains why this sentence still feels heavy, but heaviness is not the same as error.
The way to test a long sentence is to find its joins. Every place two complete thoughts meet must have one of the three legal repairs. If every join is legal, the sentence is a run-on only in the sense that it is long, which is a style problem, not a grammar problem.
A five-minute check
Take a draft and mark every period, semicolon, and conjunction. Then read between the marks. If you find two subjects and two verbs with nothing between them but a comma, you have a splice. If you find a clause starting with "because," "which," "although," or "when" and ending with a period, you have a fragment.
The grammar checker catches many of these, but it works from patterns and you work from meaning. When the tool and your ear disagree, trust the ear and check the rule.
A sentence is a promise that a thought will finish. Run-ons keep promising. Fragments never deliver. The three repairs are the same three moves in both directions, and once you have them, you will never need to count words to find the error again.