1. A common sense approach to comma usage.
Commas: The 7 Basic Rules
A common sense approach to comma usage.

2. Rule 1
Put a comma before FANBOYS (coordinating conjunctions) when they connect two independent clauses.

3. Examples
John looked up an old friend on the Internet, and then he ed her.
Mary wanted to go to the mall, but her father reminded her that she had promised to wash the car.
I will not vote for that candidate, for I am a died-in-the-wool Democrat.
You had better pay your car note, or the finance company will repossess your car.
You had better pay your car note, or the finance company will repossess your car, and you will have no way of getting to work.

4. Warning She wanted to go to college but didn’t have enough money.
Be sure the FANBOYS do connect two or more independent clauses. The following sentence is merely one independent clause with one subject and two verbs. Therefore no comma should be used.
She wanted to go to college but didn’t have enough money.

5. Rule 2
Put a comma between three or more items in a series. Think about dates and addresses as being "items in a series."

6. Examples We rang the doorbell, banged on the door, and shouted.
He ordered pie, cake, and ice cream.
On January 5, 2002, we will begin a new semester.
We will move to 165 North Avenue, Chicago, Illinois, in October.

7. Rule 3
Put a comma after introductory adverbial clauses and verb phrases (participials and infinitives).

8. Examples
To get a decent mortgage rate, Wayne went with an Internet company.
Grinning like the Cheshire cat, Mary accepted her high school diploma.
When Mary received her high school diploma, she was grinning like the Cheshire cat.
Because Mary was grinning like the Cheshire cat, people thought she was crazy.

9. Rule 4
Put commas around nouns of direct address.

10. Examples I hope you’re happy, John. John, I hope you’re happy.
I know, Mary, that you will do a good job.

11. Rule 5
Put commas around expressions that interrupt the flow of the sentence.

12. Examples I will, however, take your suggestion under advisement.
In the future, of course, you’ll want to make sure you get to class on time.
It is to your advantage, I think, to get to class on time.

13. Rule 6
Put commas around nonrestrictive (unnecessary) clauses and phrases.

14. Examples
The Volkswagen, which we bought last summer, has been a great little car.
The Magic Flute, an opera composed by Mozart in 1791, is still performed today.
Madge Diehard, whom I voted for in the last election, was soundly defeated.

15. Rule 7
Use commas to prevent misreadings.

16. Examples
Before she had believed that everything would work out well; now she was no so sure.
Before, she had believed that everything would work out well; now she was no so sure.
To John Harrison had been an idol.
To John, Harrison had been an idol.
The morning after the policeman came to the door.
The morning after, the policeman came to the door.