Use a comma before a coordinating conjunction in a compound sentence.

You'll have to define coordinating conjunctions and compound sentences. We got you.

Coordinating conjunctions are summed up as FANBOYS. No, this idea wasn't cooked up by a bunch of One Direction fans. FANBOYS represents the following: for, and, nor, but, or, yet, so.

Got it? Okay, on to the compound sentence. A compound sentence is created when you take two simple sentences and plug them together with one of the FANBOYS. Example:

Simple sentence number 1: I went to the store.
Simple sentence number 2: I bought a candy bar.
Compound sentence: I went to the store and I bought a candy bar.

Who's hungry?