When do we use a comma just before the conjunction "and" ?

Answer 1

There's two instances I can think of. First is in a list of more than two items. For example: "I'm going out to eat on Monday, Tuesday, and Wednesday." The other is when you have two complete sentences joined by "and". For instance: "I went to the hardware store, and I returned home." If the second half is not a complete sentence, then don't use a comma: "I went to the hardware store and returned home."

Answer 2

Several other people have mentioned the correct usage, but as a writing instructor, I would like to point out a frequent misuse I see.

If you have a compound object (predicate), "and" is NOT preceded by a comma (the * indicates an incorrect place to put a comma, because "fixed dinner" is not a complete sentence):

When Julie came home from school, she walked the dog* and fixed dinner.

(Yes, this does follow logically from those who have said already that a comma must separate two clauses connected with "and," but I just had to add this negative example :) )

Answer 3

We use a comma before AND when we are combining two complete sentences.
For example,
I want to go to the show, AND Tony wants to stay home.
Mary ate pie, AND Bob ate Jello.
Each sentence has a subject and a verb, and they can be separated into 2 individual sentences.
You also use a comma before AND when you are using it in a series of 3 or more items.
For example, I like coffe, tea, and Pepsi.
Tom went to Sear's, Walmart, and Target for gifts.
These rules also apply to ,but and ,or.

Answer 4

It is called a comma splice. Without it, you have a run-on sentence.

Answer 5

when it separates two complete sentences and in a list.

Answer 6

It's to separate two clauses.

Answer 7

you use it before the conjunction...