Commas used in a list or series of items are known as serial commas. In a list of three or more items, it is best to use a comma after each item except for the last. The last item should have a conjunction preceding it and does not require a comma.
Commas are only used in nonessential clauses. A clause that is not imperative to the meaning of the sentence should be set between two commas. If a clause is essential to the meaning of a sentence, commas should be left out.
Commas are used when attributing and setting up quotes. If the attribution appears before the quote, the comma should be placed outside the quotation, just before the first quotation mark. If an attribution appears after the quote, the comma needs to be placed within the quotation, immediately before the final quotation mark.
If the word "and" could be placed between adjectives in a list, then commas are needed to separate the adjectives. However, if it would obscure the meaning of the sentence or sound awkward to place "and" between adjectives, no commas are needed.
Two or more independent clauses should never be joined with just a comma; however, they can be joined with a comma and a conjunction. If the two clauses are related, it is acceptable to use a semicolon. If the two clauses are not related, they should be separated into two distinct sentences.
In a written date, a comma should always be placed between the day and the year. In a sentence, a comma should also be placed between the year and the following word.