«Is this a run-on sentence?» — how to tell
A run-on sentence occurs when two or more independent clauses (complete thoughts with their own subject and verb) are joined without appropriate punctuation or connecting words. Contrary to popular belief, a run-on sentence is a punctuation fault, not a length fault.
A 50-word sentence with proper semicolons, commas, and conjunctions is completely grammatical. Meanwhile, a six-word sentence like «I wrote it, he read it» is a classic run-on (comma splice).
The two main types: comma splices vs fused sentences
Run-on sentences typically take one of two forms:
- Comma splice: Two complete sentences joined only by a bare comma without a coordinating conjunction. Example: «The deadline moved, we rewrote the abstract.» English grammar does not allow a comma alone to join two independent clauses.
- Fused sentence (run-together sentence): Two complete sentences joined with no punctuation at all. Example: «The deadline moved we rewrote the abstract.»
Why comma splices and run-ons are so common for ESL writers
The comma splice is one of the most reliable first-language transfer errors in English. Languages like Spanish, Russian, French, Italian, Portuguese, and Chinese allow commas between clauses where English strictly requires a full stop, semicolon, or coordinating conjunction. Writers are applying a valid rule from their native language that does not translate to English.
Because each clause is spelled correctly and grammatically valid in isolation, standard spell-checkers miss run-ons. If your sentences are also bloated with filler words, check them with our wordiness checker or test overall flow with the naturalness checker.
Four ways to fix a run-on sentence (before & after)
Consider the run-on sentence: «The experiment failed, we decided to modify the protocol.» Here is how to fix it:
- 1. Add a period (two sentences):
Before: «The experiment failed, we decided to modify the protocol.»
After: «The experiment failed. We decided to modify the protocol.» (Best for academic clarity and punchy delivery). - 2. Use a semicolon:
Before: «The results were unexpected, they opened new research avenues.»
After: «The results were unexpected; they opened new research avenues.» (Best when two ideas are closely linked). - 3. Add a comma + coordinating conjunction (FANBOYS):
Before: «The data was corrupted, we had to re-run the script.»
After: «The data was corrupted, so we had to re-run the script.» - 4. Subordinate one clause:
Before: «The deadline was moved, the team took a break.»
After: «Because the deadline was moved, the team took a break.» (Best for showing causality and varying sentence structure).
If you want whole paragraphs restructured automatically, Diglot's editor and paraphrasing tool handle complex multi-clause rewrites effortlessly.
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
- Can this checker miss a run-on sentence?
- Yes. It uses browser-based rules for common comma splices and some long clause joins, not a full grammatical parser. Short fused sentences and unusual syntax may be missed. Check whether each clause can stand alone and review the punctuation yourself.
- Is this a run-on sentence?
- To test if a sentence is a run-on, look for two independent clauses (parts that could each stand alone as full sentences). If they are joined by only a comma (comma splice) or by no punctuation at all (fused sentence), it is a run-on sentence. Paste it into the tool above to check instantly.
- What is the difference between a comma splice and a fused sentence?
- A comma splice joins two independent clauses with just a comma («I arrived early, the doors were locked»). A fused sentence joins them with no punctuation whatsoever («I arrived early the doors were locked»). Both are types of run-on sentences.
- How do you fix a comma splice or run-on sentence?
- You can fix a run-on in four ways: 1) split it into two sentences with a period, 2) connect the clauses with a semicolon, 3) use a comma followed by a coordinating conjunction (for, and, nor, but, or, yet, so), or 4) add a subordinating word like «because» or «although».
- Is a long sentence always a run-on sentence?
- No. Sentence length does not determine whether a sentence is a run-on. A 60-word sentence with proper conjunctions and dependent clauses is grammatically sound, while a short 6-word sentence like «She left, he stayed behind» is a comma splice run-on.
- Is my text uploaded or stored anywhere?
- No. The run-on sentence checker runs 100% in your browser using client-side JavaScript. Your text is private and never leaves your device.
