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“Is this a run-on sentence?” is one of the most common grammar questions, and it is easy to get wrong because a run-on is not about length. Below are fifteen sentences. Decide for each one: correct, comma splice, or fused sentence. The answers are further down.
The two-question test
Before you start, here is the test used throughout the answers.
- Can each side stand alone as a full sentence? If not, it is not a run-on (it may be a dependent clause, which is fine).
- What joins them? If the answer is “nothing” (fused sentence) or “just a comma” (comma splice), it is a run-on. If the answer is a full stop, a semicolon, a colon, a comma plus and/but/so/or, or a subordinating word like although or because, it is correct.
The 15 sentences
- I wanted to go out, it was raining.
- She studies every night she wants to pass the exam.
- The experiment failed; we repeated it with new samples.
- Although the data were incomplete, the team published the results.
- He is tired, however, he keeps working.
- The city grew quickly and the roads could not cope and the council did nothing.
- My brother lives in Madrid he works as a nurse.
- I like coffee, and my sister likes tea.
- The results were clear, they supported the hypothesis.
- It was late so we left.
- Please send the report, I will review it tomorrow.
- Students who plan their essays, write faster.
- The study was small, therefore the results should be treated with caution.
- She said she was ready, then she left.
- We tested three methods: the first was fast, the second was accurate, and the third was cheap.
Answers
| # | Verdict | Why | Fix |
|---|---|---|---|
| 1 | Comma splice | Two full sentences joined only by a comma | I wanted to go out, but it was raining. |
| 2 | Fused sentence | Two full sentences with nothing between | She studies every night because she wants to pass the exam. |
| 3 | Correct | A semicolon joins two related full sentences | None needed |
| 4 | Correct | A dependent clause followed by a main clause | None needed |
| 5 | Comma splice | ”However” is not a conjunction, so a comma cannot join the clauses | He is tired; however, he keeps working. |
| 6 | Grammatical but stringy | Joined by “and”, so not a run-on in the strict sense, but three clauses in a row read poorly | The city grew quickly. The roads could not cope, and the council did nothing. |
| 7 | Fused sentence | Two full sentences with nothing between | My brother lives in Madrid, where he works as a nurse. |
| 8 | Correct | Comma plus “and” joins two full sentences | None needed |
| 9 | Comma splice | Two full sentences joined only by a comma | The results were clear: they supported the hypothesis. |
| 10 | Correct | ”So” is a coordinating conjunction; the comma before it is recommended but optional in short sentences | It was late, so we left. |
| 11 | Comma splice | An instruction and a statement, each a full sentence, joined by a comma | Please send the report. I will review it tomorrow. |
| 12 | Not a run-on | The error is a different one: a comma between the subject and the verb | Students who plan their essays write faster. |
| 13 | Comma splice | ”Therefore” does not join clauses | The study was small; therefore, the results should be treated with caution. |
| 14 | Comma splice | ”Then” does not join clauses, though narrative writing sometimes does this on purpose | She said she was ready, and then she left. |
| 15 | Correct | A colon introduces a list of clauses, and the last one is joined by “and” | None needed |
Why these mistakes happen
A few patterns explain most of the errors above.
- “However”, “therefore”, “then”, “also” and “thus” look like conjunctions but are not. They are adverbs. A comma plus one of them still leaves two sentences stuck together.
- Your first language may punctuate differently. In some languages a comma between short, closely related clauses is normal. English expects a stronger boundary.
- Fast typing joins what you were thinking as one thought. Reading aloud finds many of these: where you take a breath is often where the sentence boundary belongs.
Five ways to fix any run-on
- Full stop. Split into two sentences: “The results were clear. They supported the hypothesis.”
- Semicolon. Use it when the two sentences are closely related: “The results were clear; they supported the hypothesis.”
- Comma plus coordinating conjunction. Use for, and, nor, but, or, yet, so: “The results were clear, and they supported the hypothesis.”
- Subordinating word. Make one clause depend on the other: “Because the results were clear, we accepted the hypothesis.”
- Colon. Use it when the second clause explains the first: “The results were clear: they supported the hypothesis.”
The five fixes are covered with longer examples in our guide to sentence fragments and run-ons.
Check your own text
Paste a paragraph into the free run-on sentence checker to see which sentences look like comma splices or fused sentences. The sentence structure checker shows the shape of a paragraph, including sentences that stop short and repeated sentence openers. If you draft in more than one language, the weave editor lets you write the idea in your first language and resolve it into clean English one sentence at a time.
