- Run-ons join complete sentences without proper punctuation or a conjunction.
- A comma splice is a run-on that uses only a comma.
- Fix with a period, semicolon, comma+FANBOYS, or subordination — then verify tool suggestions.
What is a run-on sentence?
A run-on joins two or more independent clauses (complete sentences) without the punctuation or conjunction they need. A comma splice is a common run-on that joins them with only a comma.
| Type | Problem | Example |
|---|---|---|
| Fused run-on | No punctuation between clauses | The draft was late I still submitted it. |
| Comma splice | Only a comma between clauses | The draft was late, I still submitted it. |
| Correct | Period, semicolon, or comma + coordinating conjunction | The draft was late, but I still submitted it. |
Five reliable fixes
- Period: The draft was late. I still submitted it.
- Semicolon: The draft was late; I still submitted it. (Clauses must be closely related.)
- Comma + FANBOYS: for, and, nor, but, or, yet, so — The draft was late, but I still submitted it.
- Subordinating conjunction: Although the draft was late, I still submitted it.
- Em dash or colon when the second clause explains the first (use sparingly in formal essays).
See also colon vs semicolon.
Self-check before any tool
- Read the sentence aloud. If you need a full stop in your voice, the page probably does too.
- Cover the sentence and ask: could this stand alone before the comma? And after?
- Search your draft for “, however” and “, therefore” — those adverbs cannot join clauses with a comma alone (“…late; however, I submitted it”).
Which tools help (and how to use them)
Paste a suspect paragraph into Grammarly Free or LanguageTool. Accept only fixes that match the rules above. Hemingway Editor will not label “run-on”, but it highlights long sentences worth splitting. No checker catches every fused sentence, especially if both halves are short.
Practice quiz
Decide whether each item is OK, a fused run-on, or a comma splice — then open the answer.
1. The experiment failed the hypothesis was wrong.
Fused run-on. Fix: “The experiment failed. The hypothesis was wrong.” or “The experiment failed because the hypothesis was wrong.”
2. The experiment failed, the hypothesis was wrong.
Comma splice. Add a period, a semicolon, or “so/because”.
3. The experiment failed, so the hypothesis was wrong.
OK. Comma + coordinating conjunction.
4. Although the experiment failed, the team published a note.
OK. Dependent clause + comma + independent clause.
5. The results were clear, however the sample was small.
Comma splice. Use a semicolon before however: “clear; however, the sample was small.”
Next: skim best grammar checker for essays or open a profile linked in the tools section below.
Frequently asked questions
What is the best run-on sentence checker?
A grammar checker such as Grammarly Free or LanguageTool helps, but you still need the rule. Read aloud and test whether each half can stand alone.
Is a comma splice a run-on?
Yes. A comma splice is a run-on that uses only a comma between complete sentences.
Can I fix a run-on with a semicolon?
Yes, when the two clauses are closely related and both could be full sentences.
Why did my checker miss a run-on?
Short fused sentences and stylistic fragments confuse many tools. Human proofreading still matters.
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