A fused sentence (also known as a run-on) is a specific type of grammatical error that occurs when two independent clauses are joined together without any punctuation or coordinating conjunctions between them.
Think of it like two cars colliding because there was no stop sign or traffic light to separate them.
How it Happens
An independent clause is a group of words that has a subject and a verb and can stand alone as a complete sentence. A fused sentence happens when you smash two of these together.
The Error:
The sun was incredibly bright I forgot my sunglasses.
Clause 1: The sun was incredibly bright.
Clause 2: I forgot my sunglasses.
The Problem: There is no "divider" between "bright" and "I."
How to Fix It
There are four primary ways to repair a fused sentence. Choosing the right one depends on the "flow" you want for your writing.
1. Use a Period
This is the simplest fix. Turn the two clauses into two separate sentences.
Fix: The sun was incredibly bright**.** I forgot my sunglasses.
2. Use a Semicolon
Use this if the two ideas are closely related and you want to maintain a strong connection between them.
Fix: The sun was incredibly bright**;** I forgot my sunglasses.
3. Use a Comma + Coordinating Conjunction (FANBOYS)
Use a comma followed by For, And, Nor, But, Or, Yet, or So.
Fix: The sun was incredibly bright**, so** I forgot my sunglasses.
4. Use a Subordinating Conjunction
Turn one of the clauses into a dependent clause. This often makes your writing sound more sophisticated.
Fix: Because the sun was incredibly bright, I forgot my sunglasses.
Fused Sentence vs. Comma Splice
It is easy to confuse these two, as they are both "run-on" errors:
Fused Sentence: Two clauses with nothing between them.
Comma Splice: Two clauses joined by only a comma (which isn't strong enough to hold them together).

Sentence Type	Example
Fused	I love coffee I drink it every morning.
Comma Splice	I love coffee, I drink it every morning.
Corrected	I love coffee**;** I drink it every morning.