When the comma goes BEFORE "and":
* In a series of three or more items:
* "I like apples, oranges, and bananas."
* "She went to the store, bought groceries, and cooked dinner."
* When "and" joins two independent clauses:
* "The sun was shining, and the birds were singing."
* "He finished his homework, and he went to bed."
When the comma goes AFTER "and":
* When "and" joins two words or phrases that aren't independent clauses:
* "He was tired and hungry."
* "They went to the park and played frisbee."
Let's look at a tricky example:
* "I went to the store, and I bought some milk."
Here, the comma is correct BEFORE the "and" because it's joining two independent clauses. Each clause could stand alone as a sentence:
* "I went to the store."
* "I bought some milk."
The "Oxford Comma"
Sometimes you'll see a comma before "and" in a series of items, even when there are only two items. This is called the "Oxford comma," and it's a matter of style preference.
* "I like apples, and bananas." (Oxford comma)
* "I like apples and bananas." (No Oxford comma)
In conclusion:
Whether you use a comma before "and" depends on the grammatical structure of your sentence. Use a comma before "and" when joining independent clauses or in a series of three or more items.