A comma is used before a coordinating conjunction that links independent clauses. An independent clause contains a subject and a verb and can stand on its own as a sentence.
One of the ways we use commas is to join two independent clauses with a coordinating conjunction. When a clause makes sense by itself and can stand as an independent sentence, it’s called an independent clause.
For example: The rain burst from the clouds [independent clause] and the night sky was filled with lightning [independent clause].
In this sentence there are two independent clauses. That is, the words on either side of the “and” form complete sentences. Therefore, we need a comma before the coordinating conjunction “and.”
Corrected example: The rain burst from the clouds, and the night sky was filled with lightning.
On the other hand, when a clause depends on another clause for its meaning and can’t stand on its own, it’s called a dependent clause. When a dependent clause or a phrase follows a coordinating conjunction, do not use a comma before the coordinating conjunction.
For example: The rain burst from the clouds [independent clause] and soaked the baseball field [dependent clause].
In this sentence, what follows “and” isn’t a complete sentence. It doesn’t have a subject, and it doesn’t make sense on its own. Imagine someone coming into a room and saying, “soaked the baseball field.” You wouldn’t know what or who soaked the field. Is the speaker saying he soaked the field? Did a flash flood sweep through? The dependent clause depends on the independent clause that comes before it in the sentence.
Here’s a list of coordinating conjunctions:
If you have trouble remembering these, here’s a tip: You can spell the word “FANBOYS” using the first letter of each of the coordinating conjunctions.