Run-On Sentences and Comma Splices: How to Spot and Fix Them

CorrectNow Editorial Team · 6 min read

A run-on sentence joins two or more complete sentences without the punctuation or linking word they need. The result can be hard to follow, because the reader cannot see where one idea ends and the next begins.

This guide explains the two main types, the fused sentence and the comma splice, clears up a common myth about long sentences, and shows several ways to fix them.

What counts as a complete sentence

To spot a run-on, you need to recognise an independent clause. An independent clause has a subject and a verb and expresses a complete thought: The shop closed early. We went home.

When two independent clauses meet, they need something between them: a full stop, a semicolon, or a comma with a coordinating conjunction such as and, but or so. Without that, you have a run-on.

Fused sentences: no punctuation at all

A fused sentence runs two independent clauses together with nothing between them. It is the most obvious kind of run-on and tends to appear in quick drafts and messages.

Incorrect: The train was delayed we missed the start of the show. – Correct: The train was delayed, so we missed the start of the show. (So shows that the second idea is a result of the first.)
Incorrect: I called the office nobody answered. – Correct: I called the office. Nobody answered. (Two short sentences are often the clearest fix.)

Comma splices: a comma doing a full stop's job

A comma splice joins two independent clauses with only a comma. It looks more finished than a fused sentence, which is why it is so common. A comma can separate items and phrases, but in standard writing it cannot join two complete sentences on its own.

Fiction writers sometimes use comma splices on purpose for rhythm, especially with very short clauses. In business, academic and most everyday writing, they are treated as errors.

Incorrect: The report is finished, I will send it tomorrow. – Correct: The report is finished; I will send it tomorrow. (A semicolon links two closely related complete sentences.)

Five ways to fix a run-on sentence

Choose the fix that shows how the ideas relate. A full stop keeps them separate, a semicolon shows they are closely connected, and a conjunction names the relationship, such as contrast with but or result with so.

Use a full stop: The report is finished. I will send it tomorrow.
Use a semicolon when the ideas are closely linked: The report is finished; I will send it tomorrow.
Add a comma and a coordinating conjunction: The report is finished, and I will send it tomorrow.
Make one clause dependent with a word like because, although, when or since: Since the report is finished, I will send it tomorrow.
Rewrite the sentence: I will send the finished report tomorrow.

Watch out for however, therefore and then

Words such as however, therefore, moreover and otherwise are adverbs, not coordinating conjunctions. They cannot join two independent clauses with only a comma. Use a semicolon or a full stop before them, and usually a comma after.

Then causes a similar problem. Many editors treat a comma followed by then, joining two full sentences, as a comma splice. Adding and is the simplest fix.

Incorrect: The price is high, however the quality is excellent. – Correct: The price is high; however, the quality is excellent. (However needs a semicolon or a full stop before it here.)
Incorrect: We checked the data, then we wrote the summary. – Correct: We checked the data, and then we wrote the summary. (And does the joining; then shows the order.)

Long sentences are not always run-ons

A run-on is about structure, not length. A long sentence can be perfectly correct if its clauses are properly joined, and a very short sentence can be a run-on: It's late let's go is a fused sentence that needs a full stop or a semicolon after late.

Still, very long sentences can tire readers. If a correct sentence is hard to follow, splitting it can help, even though it is not an error.

When you proofread, look at each comma and ask whether a complete sentence sits on both sides of it. A grammar checker such as CorrectNow can flag likely comma splices, explain the fix, and correct the punctuation without rewriting your sentences.

Frequently asked questions

Is a run-on sentence just a long sentence?

No. A run-on joins complete sentences incorrectly. A long sentence with properly joined clauses is not a run-on, and a short one can be.

What is the difference between a run-on and a comma splice?

A comma splice is one type of run-on, where a comma alone joins two complete sentences. A fused sentence is the other type, with no punctuation at all. Some people use run-on to mean only fused sentences.

Can I fix any comma splice with a semicolon?

A semicolon works when both sides are complete sentences and the ideas are closely related. If the ideas are only loosely connected, a full stop usually reads better.

Is a comma splice ever acceptable?

In fiction and informal writing, short balanced clauses are sometimes joined with commas for effect. In formal, academic and business writing, avoid comma splices.

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