Paste your essay, email, or chapter to flag comma splices (such as ', however' or ', therefore'), multi-clause chains, and breathless sentences over 35 words. Everything runs locally in your browser.
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Every run-on sentence or comma splice happens when two independent clauses (complete thoughts with a subject and verb) collide without strong enough punctuation. Pick one of these four fixes:
| Fix Method | Before (Flagged) | After (Corrected) | When to Use |
|---|---|---|---|
| 1. Period + Capital Letter | The deadline passed at midnight, however the editor accepted the draft. | The deadline passed at midnight. However, the editor accepted the draft. | Best when the two clauses make distinct points or the sentence already exceeds 25 words. |
| 2. Semicolon (;) | She cut the opening chapter, therefore the pacing improved immediately. | She cut the opening chapter; therefore, the pacing improved immediately. | Best before conjunctive adverbs (however, therefore, moreover, nevertheless). |
| 3. Comma + Coordinating Conjunction (FANBOYS) | He rewrote the pitch three times, he still missed the hook. | He rewrote the pitch three times, yet he still missed the hook. | Use for, and, nor, but, or, yet, so to show the exact logical relationship. |
| 4. Subordinate One Clause | The novel was 110,000 words long, and the agent passed on it, so the author cut 20,000 words. | After the agent passed on the 110,000-word novel, the author cut 20,000 words. | Best for fixing chained 'and... so... but' sentences over 35 words. |
In our 15-error grammar checker benchmark, the single most common punctuation mistake in business emails and academic essays is placing a comma before however, therefore, moreover, furthermore, nevertheless, or consequently when joining two complete clauses.
Only the seven coordinating conjunctions (For, And, Nor, But, Or, Yet, So) can legally join two independent clauses after a comma. Anything else requires a semicolon or a period. For automatic one-click sentence splitting inside Google Docs and Word, compare Grammarly Free vs Pro or read our ProWritingAid review.
A fused (run-on) sentence joins two independent clauses with no punctuation at all ('I finished the draft I sent it to my editor'). A comma splice joins two independent clauses with only a bare comma ('I finished the draft, I sent it to my editor' or 'I finished the draft, however my editor was away'). Both are structural punctuation errors.
Length alone does not make a sentence grammatically wrong, but readability studies show reader comprehension drops sharply past 30 to 35 words. This checker flags sentences of 35+ words and sentences that chain multiple coordinating conjunctions so you can split them cleanly.
Words like 'however', 'therefore', 'furthermore', and 'moreover' are conjunctive adverbs, not coordinating conjunctions. Joining two complete sentences with ', however' creates a comma splice. Place a semicolon or period before 'however' and a comma after it ('; however,').
Grammarly Free catches basic missing commas, but its full-sentence splits for convoluted or run-on sentences are locked behind Grammarly Pro ($12/month billed annually). You can check run-ons and comma splices free in the tool above, or compare Grammarly Pro and ProWritingAid in our review.
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