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

Published: 2026-05-15 · 6 min read

Run-on sentences are one of the most common writing errors in English. They happen when two or more complete thoughts are joined together without the right punctuation or connecting words. The result is a sentence that feels rushed, confusing, and grammatically incorrect.

The good news? Run-ons are easy to fix once you know what to look for. This guide covers everything you need: what run-on sentences are, the different types, and four reliable methods to fix them.

What Is a Run-On Sentence?

A run-on sentence occurs when you join two independent clauses (groups of words that could stand alone as complete sentences) without proper punctuation or a conjunction.

Example of a run-on:
The sun was setting we decided to go for a walk.

Both "The sun was setting" and "We decided to go for a walk" are complete sentences. Smashing them together without any connector creates a run-on.

Comma Splices: The Most Common Run-On

A comma splice is a specific type of run-on where two independent clauses are joined with only a comma. This is arguably the most frequent punctuation mistake in informal writing.

Example of a comma splice:
The results were inconclusive, the team decided to run another test.

The comma here is too weak to hold two complete sentences together. You need either a period, a semicolon, or a conjunction.

Four Reliable Ways to Fix a Run-On Sentence

Every run-on sentence can be repaired with one of these four methods. Which one you choose depends on the relationship between your two ideas.

Method 1: Use a Period (Make Two Sentences)

This is the simplest fix. If the two clauses are distinct ideas that don't need to share a sentence, split them apart.

Before: The experiment took three hours the results were worth the wait.
After: The experiment took three hours. The results were worth the wait.

Best for: When the two ideas are loosely related and you want a natural pause. This method is almost always grammatically correct, so it's a safe fallback.

Method 2: Use a Semicolon

A semicolon joins two closely related independent clauses without a conjunction. It signals to the reader that the second clause expands on or contrasts with the first.

Before: Sarah loves hiking Mark prefers the gym.
After: Sarah loves hiking; Mark prefers the gym.

Best for: When the two clauses are closely connected in meaning and roughly equal in importance. A semicolon creates a stronger link than a period but a lighter connection than a conjunction.

Method 3: Add a Comma + Coordinating Conjunction

The seven coordinating conjunctions (for, and, nor, but, or, yet, so) can combine with a comma to join independent clauses. Each conjunction carries a specific meaning.

Before: I wanted to attend the conference tickets were sold out.
After: I wanted to attend the conference, but tickets were sold out.

Choose your conjunction carefully:

• And — adds similar information
• But — shows contrast
• So — indicates a result
• Or — presents an alternative
• Yet — shows contrast (stronger than "but")
• For — explains a reason (formal)
• Nor — adds a negative alternative

Best for: When you want to show a specific relationship between two ideas — contrast, cause, addition, or choice.

Method 4: Use a Subordinating Conjunction

Subordinating conjunctions (because, although, while, since, unless, when, after, etc.) turn one clause into a dependent clause, creating a complex sentence.

Before: The network went down we lost two hours of work.
After: Because the network went down, we lost two hours of work.

Best for: When one idea is logically subordinate to the other — cause/effect, time relationships, conditions, or concessions.

Run-On vs. Long Sentence: What's the Difference?

A long sentence is not necessarily a run-on. You can write a grammatically correct sentence that spans several lines, as long as you use proper punctuation and structure.

Long but correct: After the storm passed, the team surveyed the damage, documented the losses, and began the cleanup process, which took three full days to complete.

Run-on (same ideas, wrong structure): The storm passed the team surveyed the damage they documented the losses and began the cleanup process it took three full days.

The key distinction: grammatically well-constructed long sentences use dependent clauses, conjunctions, and punctuation to guide the reader. Run-ons throw ideas together and leave the reader to sort them out.

More Before-and-After Examples

Business writing:
Run-on: Our Q3 revenue exceeded projections we attribute this to the new product launch.
Fixed (period): Our Q3 revenue exceeded projections. We attribute this to the new product launch.
Fixed (semicolon): Our Q3 revenue exceeded projections; we attribute this to the new product launch.
Fixed (conjunction): Our Q3 revenue exceeded projections, and we attribute this to the new product launch.
Fixed (subordination): Because our Q3 revenue exceeded projections, we attribute this to the new product launch.

Academic writing:
Run-on: The participants were divided into two groups one received the treatment the other received a placebo.
Fixed: The participants were divided into two groups: one received the treatment, while the other received a placebo.

Creative writing:
Run-on: She opened the door the room was completely empty except for a single chair in the corner.
Fixed: She opened the door, and the room was completely empty — except for a single chair in the corner.

Common Misconceptions

"Every long sentence is a run-on." No. A sentence can be long and grammatically perfect if it uses proper subordination and punctuation.

"Two sentences always need a conjunction." Not true. A period or semicolon works perfectly well when the relationship between the ideas is clear.

"Starting with 'because' creates a fragment." Only if you don't complete the thought. "Because it was raining" is a fragment. "Because it was raining, we cancelled the picnic" is a correct complex sentence.

The Bottom Line

Run-on sentences are easy to fix once you recognize them. The four methods — period, semicolon, comma plus conjunction, and subordinating conjunction — cover every situation you'll encounter. The best method depends on the relationship between your ideas and the tone you want.

When in doubt, split the sentence with a period. You can never go wrong with two clear, complete sentences.

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