What Is a Run-On Sentence?
A run-on sentence occurs when two or more independent clauses are joined without the correct punctuation or coordinating conjunction. It doesn't mean the sentence is too long — it means two complete thoughts have been fused together improperly.
Here's a classic example:
"I went to the store I forgot my wallet."
Both "I went to the store" and "I forgot my wallet" are complete thoughts. Mashing them together without any connector creates a run-on.
The Two Main Types of Run-Ons
1. Fused Sentences
A fused sentence happens when two independent clauses are joined with no punctuation at all, like the example above.
2. Comma Splices
A comma splice occurs when two independent clauses are separated by only a comma — which isn't strong enough punctuation to do the job.
"She studied all night, she still felt unprepared."
That comma is doing too much work. It needs help.
Four Ways to Fix a Run-On Sentence
- Use a period. Split the two clauses into separate sentences.
"I went to the store. I forgot my wallet." - Use a semicolon. If the two ideas are closely related, a semicolon connects them correctly.
"I went to the store; I forgot my wallet." - Add a coordinating conjunction. Use one of the FANBOYS (for, and, nor, but, or, yet, so) with a comma.
"I went to the store, but I forgot my wallet." - Use a subordinating conjunction. Turn one clause into a dependent clause to show the relationship between ideas.
"When I went to the store, I realized I had forgotten my wallet."
Why Do Run-Ons Happen?
Run-ons are so common because writers are often thinking faster than they're typing. Your brain knows the ideas are connected, so it skips the grammatical signpost that tells the reader where one thought ends and another begins.
They're especially common in:
- First drafts written quickly
- Informal writing and emails
- Writing by non-native English speakers who follow different sentence-boundary conventions
A Quick Test to Catch Run-Ons
Read your sentence aloud. If you naturally pause in the middle and both halves feel like complete thoughts, you may have a run-on. Ask yourself: could each part stand alone as its own sentence? If yes, check your punctuation.
You can also try the "period test": place a period wherever you feel a natural pause. If both pieces are grammatically complete sentences, the original was a run-on.
Common Mistakes When Fixing Run-Ons
Watch out for these traps:
- Using a comma alone — that just creates a comma splice, which is still an error.
- Adding a conjunctive adverb (however, therefore, moreover) without a semicolon — "I was tired, however I kept working" is still a comma splice. It should be "I was tired; however, I kept working."
Practice Makes Permanent
The best way to stop writing run-ons is to slow down during revision. Read each sentence independently, check that it contains exactly one independent clause (or more joined correctly), and you'll eliminate this error from your writing for good.