There are 11 simple rules that govern the comma in AP style. Be aware that this is a particular style. Other styles have different rules for comma use.
1) When the last item in a series is connected by a coordinating
conjunction (e.g., and, or, but, nor, for, yet, so), do not use
a comma before the conjunction.
- I enjoy golf, football and boxing.
2) Use a comma to separate two independent clauses connected by
a coordinating conjunction.
- The UO football team won the game, and the Ducks are going to
the Rose Bowl.
3) Remember: A compound predicate (two or more verbs serving the
same subject) does not need a comma.
- The man voted today and hoped his candidate would win.
4) Use commas following introductory clauses and phrases and other
clauses and phrases that would be confusing without commas.
- In the hassles and headaches of daily life at the University,
it is easy to forget how privileged we are to attend college.
- Although she had always been afraid to fly, she loved her flight in a small plane.
- Every day, journalists report the news.
5) Use commas to set off non-restrictive (non essential) clauses,
phrases and modifiers from the rest of the sentence.
- The lawn mower that is broken is in the garage.
- (Restrictive: Indicates more than one lawn mower)
- The lawn mower, which is broken, is in the garage.
- (Nonrestrictive: Adds non-essential information about the only lawn mower)
6) Use commas to separate descriptive modifiers of equal rank if the coordinating conjunction is missing.
- Tip: If you can use the adjectives interchangeably and can successfully
insert a conjunction and between them, they require a comma.
- In an angry, blunt statement, President Clinton chided his opponents.
7) Use commas to set off parenthetical expressions and nominatives of direct address.
- The test, you may recall, was easy.
- She said, "You know, Jan, that the test is today."
- "Jan, where's the car?" "Where's the car, Jan?"
8) Use commas to set off participial phrases that modify
some part of the independent clause.
- The runner quit, having cut his toe on a broken bottle.
- The judge, tired of the commotion in the courtroom, made everyone leave.
- Driven by an unquenchable desire to win, Sally often cheated.
9) Do not use a comma to separate two independent clauses that
are not joined by a coordinating conjunction. Do not use a comma to
introduce a subordinate clause.
- The test was today, we all passed. (WRONG)
- We all passed the test because it was easy. (RIGHT)
10) Do not use a comma to separate a reflexive pronoun.
- The mayor himself will be here today.
11) Do not use a comma to precede a partial quotation.
- The mayor said that his opponent was "one of the worst candidates ever to run for office."
BUT: If the quotation is a full sentence, it should be preceded by a comma.
- The mayor said, "John Smith is one of the worst candidates ever to run for office."