The Comma Splice
Recognize a comma splice when you see one.
A comma splice, also called a run-on, occurs when a writer has connected two main clauses with a comma alone. A main clause makes a complete thought, so you should not find a wimpy comma struggling to join two such powerful clauses.
The problem looks like this:
main clause + , + main clause.
Here is an example:
Fanning the slice of pizza with a napkin, Jolene waited for it to cool, she had already burned the roof of her mouth with the fried cheese sticks.
The first main clause is Jolene waited for it to cool, and the second is she had already burned the roof of her mouth with the fried cheese sticks. Notice that the two clauses have only a comma connecting them.
Fixing a comma splice is easy. All you have to do is pick one of the four available strategies.
First, you can break the error into two separate sentences, like this:
Fanning the slice of pizza with a napkin, Jolene waited for it to cool. She had already burned the roof of her mouth with the fried cheese sticks.
Another good option is to connect the two main clauses with a comma and a coordinating conjunction:
Fanning the slice of pizza with a napkin, Jolene waited for it to cool, for she had already burned the roof of her mouth with the fried cheese sticks.
You can also use a semicolon, a mark of punctuation as powerful as a period:
Fanning the slice of pizza with a napkin, Jolene waited for it to cool; she had already burned the roof of her mouth with the fried cheese sticks.
Your last option is to use a subordinate conjunction. This method reduces one of the two clauses to an incomplete thought:
Fanning the slice of pizza with a napkin, Jolene waited for it to cool since she had already burned the roof of her mouth with the fried cheese sticks.