Run-on Sentence Checker — Find Comma Splices & Fused Sentences
Paste your English to find run-on sentences — comma splices and fused clauses are flagged with the exact join that causes them, so you can see where the sentence should have ended.
The two kinds of run-on, and why only one looks wrong
A run-on is two complete sentences joined without the punctuation English requires. It comes in two shapes. The fused sentence has nothing between them at all — «The deadline moved we rewrote the abstract» — and most writers spot it on a reread.
The comma splice is the one that survives proofreading: «The deadline moved, we rewrote the abstract.» It looks punctuated, it reads smoothly, and it is still two sentences wearing one sentence's clothes. English will not let a bare comma carry that join. Four things will: a full stop, a semicolon, a comma plus a coordinating conjunction (and, but, so, or, for, nor, yet), or a subordinator that demotes one clause («Because the deadline moved, we rewrote the abstract»).
Why comma splices are a second-language pattern
The comma splice is one of the most reliable first-language transfer errors in English, because plenty of languages punctuate exactly that way. Russian, Spanish, Italian, French, Portuguese and Chinese all permit a comma between clauses that English would insist on splitting or joining explicitly. Writers are not being careless — they are applying a rule that is correct in their first language and does not carry across.
Which is why a spellchecker misses it: nothing is misspelled, and each half is grammatical on its own. It also tends to cluster, because it comes from a habit rather than a slip — find one and there are usually more in the same paragraph.
Four ways to fix a run-on, and when to use each
Take «The results were inconclusive, we repeated the experiment.»
- Full stop — «The results were inconclusive. We repeated the experiment.» Always correct, and the safest default in academic writing.
- Semicolon — «The results were inconclusive; we repeated the experiment.» Use when the two halves are tightly linked and you want to keep them in one breath.
- Comma + conjunction — «The results were inconclusive, so we repeated the experiment.» Best when the relationship matters, because the conjunction names it.
- Subordinate one clause — «Because the results were inconclusive, we repeated the experiment.» Strongest option: it makes the logic explicit and varies the sentence rhythm.
If several long sentences pile up, the underlying issue may be length rather than punctuation — the word counter flags sentences over 40 words, and the wordiness checker finds the padding that made them long.
Take it further with Diglot
This free tool runs in your browser. When you want to act on what it shows — fix the grammar, tighten the phrasing, or make your English read like a native wrote it — that is what Diglot is built for. The free tier is meaningful for daily writing, no card required.
Frequently asked questions
- What is a run-on sentence?
- Two complete sentences joined without the punctuation English requires. Either nothing separates them (a fused sentence) or only a comma does (a comma splice). Length alone does not make a run-on — a very long sentence that is correctly punctuated is not one, and a short one like «I came, I saw» is.
- What is a comma splice?
- A run-on where two independent clauses are joined by a bare comma: «It was late, we went home.» Fix it with a full stop, a semicolon, a comma plus a coordinating conjunction (and, but, so, or), or by subordinating one clause with a word like «because» or «although».
- How does the checker find them?
- It looks for a comma followed by a subject pronoun or demonstrative and then a verb — the structural signature of a comma splice — and for sentences that run long with several clause joins and no coordinating conjunction. It is a heuristic, so it can miss an unusual construction and occasionally flag a correct one; you make the final call.
- Is a long sentence always a run-on?
- No, and this is the most common misunderstanding. A run-on is a punctuation fault, not a length fault. A 60-word sentence with correct semicolons and conjunctions is fine; a six-word comma splice is a run-on. Length is a separate readability question.
