Use a comma

between a list of three or more words

To replace the word and for all but the last instance.
Up, down, left and right.

before a conjunction

1) When but or for are used.
I did my best to protect the camp, but the bears were too aggressive.

2) When and or or are used the comma is optional.
The flag is red, white, and blue. [known as the Oxford comma]
The sizes are small, medium or large.

to give additional information

1) To indicate contrast.
The snake was brown, not green, and it was quite small.

2) Where the phrase could be in brackets.
The recipe, which we hadn't tried before, is very easy to follow.

3) Where the phrase adds relevant information.
Mr Hardy, 68, ran his first marathon five years ago.

4) Where the addition is not necessary to the meaning of the sentence.
Mr Hardy, who enjoys bird watching, ran his first marathon five years ago.

5) Where the main clause of the sentence is dependent on the preceding clause.
If at first you don't succeed, give up.
Though the snake was small, I still feared for my life.

Continued on Comma (page 2)