Dr. Harold William Halbert
Coordinating conjunctions connect words, phrases, and clauses. Sometimes they need a comma before them, while other times they don’t.
FANBOYS= Complete list of coordinating conjunctions
For
And
Nor
But
Or
Yet
So
So when do you need a comma in front of them?
1. Put a comma before the coordinating conjunction if it connects two complete sentences/independent clauses.
Bob ate dinner COMMA and he went to bed.
Note that on either side of the COMMA+AND, there is a complete sentence with a subject and a verb phrase that expresses a complete thought.
Bob ate dinner.
He went to bed.
Each can stand on its own, but they can be connected with a COMMA + FANBOYS combination.
NOTE: If one side is not a complete sentence, particularly if it lacks a subject, NO COMMA IS USED.
Bob ate and went to bed.
Since “went to bed” doesn’t have a subject, it is not a complete sentence. What you have here is a pair of verbs:
Bob [verb] and [verb phrase]
Don’t make the mistake of thinking long phrases must be a complete sentence/independent clause and put a comma. Check for the subject on both sides:
Bob ate a wonderful meal consisting of shrimp and grits and then went to bed in the master bedroom of his luxury hotel suite.
It’s tempting to think there needs to a comma in front of the AND because the phrases are so long, but there is not new subject after the AND, so it’s not a potential complete sentence/independent clause. No comma is allowed.
Oxford Comma Rule
There is a major disagreement in various writing communities about the Oxford Comma. This comma appears before AND when the AND signals the final item in a series:
I like ham, tuna, and turkey sandwiches.
I like ham, tuna, and peanut butter and jelly sandwiches.
Without the Oxford Comma, one might think “peanut butter” was a separate sandwich type than “jelly.”
However, some style guides or individuals don’t use this: you should ask your professors or employers if they have a preference. For example, the Associated Press (AP) style guide for journalists prohibits the use of the Oxford Comma.