Use a comma with a coordinating conjunction when combining two independent clauses.
An independent clause is a term for a complete thought or sentence with both a subject and a verb. A coordinating conjunction is a conjunction that combines two equal elements and can combine two complete sentences. We have seven coordinating conjunctions in American English:
And
Nor
But
Or
Yet
So
You may have heard of these referred to as the FANBOYS, which is a great way to help you remember all seven of the coordinating conjunctions.
Coordinating conjunctions can connect all kinds of words and phrases, but when they are used to connect independent clauses, we must place a comma before the coordinating conjunction, as illustrated in the following example:
I am preparing for a zombie invasion, and I am building a strong zombie fort around my home.
TIP! If the coordinating conjunction does not connect two independent clauses, you would not need a comma before the coordinating conjunction, as illustrated in the following example:
I am preparing for a zombie invasion and am building a strong zombie fort around my home.