Read the rules and the examples below carefully, and check them again when you write your next composition. If you do this regularly, little by little these rules will become automatic and you won't need to look back at them! :)
(Adapted from www.eslbasics.com)
Use commas to separate words or phrases in a series of three or more.
I need to pick up bananas, tuna, bread, and ice cream from the grocery store today.
Use a comma to separate independent clauses when they are joined by and, but, for, or, nor, so, yet. You do not need to use a comma if both independent clauses are short.
We agreed to come with Sarah, but we told her that we would need to leave early.
Use commas when the relative clause is non-defining, that is, when it gives extra, unnecessary information.
Mr. Perryman, who enjoys playing golf, decided to retire in Florida.
The boy who lives next door to us is in the fourth grade.
Use commas after introductory phrases and clauses that come before the main clause. If the introductory phrase has less than three words, the comma is optional.
When the clock struck midnight, the carriage turned back into a pumpkin.
After you leave school, make sure to pick up the mail.
Next Saturday we will be getting married.
Use commas before and after "interrupting words" such as however and therefore.
I would, therefore, like to continue this conversation at another time.
Use a comma after introductory adverbs.
Fortunately, Samir had remembered to back up his files.
Unsurprisingly, Raul passed the class with flying colours.
Use commas when directly addressing a person (vocative).
Have you, Millie, considered going to college?
What do you think that means, Doctor?
Use a comma before question tags.
You're a senior in college, aren't you?
I hope you have found this entry useful! Now make sure you also check the entry When should you not use a comma: correcting a run-on sentence. Little by little we'll become punctuation experts! :)