A fused sentence is what happens when two sentences are mashed together without punctuation.
The boys played video games they watched a movie.
No, no, no, no. That's just ugly and wrong. We are blessed with the powers of punctuation. Use it for the good of humanity.
Got it? Good! Now let's talk about how to fix them.
Fixing fused sentences
1. Combine ideas.
The boys played video games and watched a movie.
Here, we simply took the main ideas from the two fused sentences above and merged them into one complete, grammatically spectacular sentence. Since we got rid of the second "they," there's no need for the comma; you only need that comma when you're combining two independent clauses or verb phrases.
2. Use a comma and a conjunction.
The boys played video games, and they watched a movie.
In this example, we separated the fused sentences with all the precision of a grammatical surgeon and then rejoined them correctly with a comma and the coordinating conjunction and. Looks good, doesn't it?
3. Use a semi-colon.
The boys played video games; they watched a movie.
Grammatically correct? Yes. But since the two parts of the sentence aren't super connected content-wise, it's not our first choice.
4. Use a period.
The boys played video games. They watched a movie.
Another fine-but-not-great example. At least there are no fused sentences, though.