Run-On Sentences and Comma Splices: 4 Ways to Fix Them

A run-on sentence joins two or more complete sentences (independent clauses) without the punctuation or connecting word needed to link them. It comes in two forms: a fused sentence, with nothing between the clauses ("I missed the bus I was late"), and a comma splice, with only a comma ("I missed the bus, I was late"). You can fix either one by adding a period, adding a semicolon, adding a comma with a conjunction, or turning one clause into a dependent clause.

What counts as a run-on

Every complete sentence needs at least one independent clause: a subject and a verb that together express a complete thought. "The store closed" is one. "We went home" is another.

Trouble starts when you put two of them side by side without a proper joint.

Fused sentence (no punctuation at all):

• ✗ The store closed early we went home.
• ✗ My laptop died I lost the last page.

Comma splice (a comma doing a job it can't do):

• ✗ The store closed early, we went home.
• ✗ My laptop died, I lost the last page.

Most style guides treat both as run-ons. Comma splices are more common in student and work writing because the comma looks like it's doing something. It isn't: a comma can separate many things, but it can't join two complete sentences on its own.

How to spot a run-on in your own writing

Run-ons are easy to write and hard to see, because when you reread, your brain fills in the pause. Try these checks:

1. Find the subject-verb pairs. Underline each verb and ask who or what is doing it. If you find two complete pairs in one sentence, check what connects them.
2. Look at every comma. Cover everything to the left of it. Could the words on the right stand alone as a sentence? Now cover the right side. Could the left side stand alone too? If both can, a comma alone isn't enough.
3. Watch for pronouns after a comma. Splices often hide in patterns like "..., it was," "..., she said," "..., this means," or "..., they were."
4. Watch for transition words. However, therefore, then, also, and otherwise often start a new independent clause, and a comma before them creates a splice.

Fix 1: Split it into two sentences

The simplest fix is a period.

• ✗ The experiment failed, we started over.
• ✓ The experiment failed. We started over.

Use this when the two ideas are fairly separate, or when the sentence was already long. Short sentences in a row can sound choppy, so if a paragraph starts to feel like a list of facts, try one of the other fixes for variety.

Fix 2: Use a semicolon

A semicolon joins two complete sentences that are closely related, without needing a connecting word.

• ✗ The first draft was too long, the second was too short.
• ✓ The first draft was too long; the second was too short.

Semicolons work best when the two clauses balance or contrast each other. They're also the right choice before a transition word such as however or therefore:

• ✗ The data looked promising, however, the sample was small.
• ✓ The data looked promising; however, the sample was small.

Don't overuse them. One or two per page is plenty in most writing.

Fix 3: Add a comma and a coordinating conjunction

The seven coordinating conjunctions are for, and, nor, but, or, yet, so (often remembered as FANBOYS). A comma plus one of these is strong enough to join two complete sentences.

• ✗ I studied all weekend, I still felt unprepared.

• ✓ I studied all weekend, but I still felt unprepared.

• ✗ The train was delayed we took a taxi.

• ✓ The train was delayed, so we took a taxi.

This fix has a bonus: the conjunction tells the reader how the ideas relate. But signals contrast, so signals a result, and simply adds. Pick the one that matches what you mean.

Fix 4: Make one clause dependent

Add a subordinating word such as because, although, when, since, if, after, while, or unless to one of the clauses. That clause can no longer stand alone, so the sentence becomes one complete sentence with a supporting part.

• ✗ It was raining, the game went ahead.

• ✓ Although it was raining, the game went ahead.

• ✗ We left early the roads were icy.

• ✓ We left early because the roads were icy.

When the dependent clause comes first, put a comma after it. When it comes second, you usually don't need one. This is often the most fluent fix, because it shows the logical connection (cause, contrast, time, condition) instead of just stacking ideas.

Which fix should you choose?

All four are correct. Mixing them across a paragraph gives your writing a more natural rhythm than relying on just one.

Long sentences are not automatically run-ons

A common misunderstanding is that any long sentence is a run-on. This one is long but correct:

When the committee finally met in March, after months of delays and two changes of chair, it approved the budget, the hiring plan, and the new grading policy in a single afternoon.

It has one independent clause ("it approved...") and everything else is attached properly. Length can still hurt readability, so if a correct sentence is hard to follow, splitting it may help. That's a style choice, though, not a grammar fix. Using active voice can also make long sentences easier to read.

Practice: fix these run-ons

Try fixing each one before reading the suggested answer. More than one fix is correct.

1. The lab closes at six, we should leave now.
2. She didn't reply to my email I sent another one.
3. The book was interesting, however it was too long.
4. Prices went up, sales stayed the same.

Suggested fixes:

1. The lab closes at six, so we should leave now.
2. Because she didn't reply to my email, I sent another one.
3. The book was interesting**;** however, it was too long.
4. Prices went up**. Still,** sales stayed the same. (Or: Although prices went up, sales stayed the same.)

Check your draft

Run-ons are among the errors a context-aware checker catches well, because the problem is structural. ZeroTell's Grammar Checker flags comma splices and run-on sentences and, if you choose explanations, says why each change was made, which helps you start spotting them yourself. When a sentence needs more than punctuation, the Sentence Rewriter can show a few clearer ways to say it.

Grammar CheckerFix grammar and phrasing

For the full set of comma rules, including when a comma is the right choice, read our comma rules guide.

Key takeaways

• A run-on is two complete sentences joined without the right punctuation.
• A comma splice uses only a comma; a fused sentence uses nothing.
• Fix it with a period, a semicolon, a comma plus FANBOYS, or a dependent clause.
• However and therefore need a semicolon or a period before them, not just a comma.
• Long doesn't mean run-on; structure does.