A comma in writing is like a pause inside a sentence when speaking. We use commas inside sentences. Commas separate parts of a sentence into logical elements. Commas have no meaning, but they help us to see the structure and therefore the meaning of the sentence.

Put a space after a comma. Do not put a space before a comma.

good spacing:
xxx, xxx
bad spacing:
xxx ,xxx
xxx , xxx

1. Use a comma between items in a series or list. In a sentence, the last two items usually do not need a comma between them as they are separated by "and". However, if one or both of the last two items are long, a comma may be useful.

2. Use a comma between three or more adjectives or adverbs.

3. For two adjectives, use a comma where you could use "and".

4. Use a comma for numbers over 999. (In English, commas separate thousands and periods separate decimals. Note that some languages use the opposite system.)

5. Use a comma for addresses, some dates, and titles following a name.

6. Use a comma before or after direct speech. Do not use a comma for reported speech.

7. Use a comma before a coordinating conjunction (for, and, nor, but, or, yet, so) to join two independent clauses. If the independent clauses are short and well-balanced, a comma is optional.

8. Use commas for parenthetical elements. A "parenthetical element" is any part of a sentence that can be removed without changing the real meaning of the sentence.

9. Use a comma after an introductory element. A comma is optional for short, simple introductory elements.

10. Sentence adverbs (words like however, unfortunately, surprisingly that modify a whole sentence) often require one or two commas, depending on their position in the sentence.

11. An adverbial clause often needs a comma when it comes at the beginning of a sentence (but not at the end of a sentence).

12. Do not use a comma to separate two complete sentences. In this case, use a full stop (period) or semi-colon.

Tara, Ram and Anthony enjoyed their holiday, which they spent in Rio Claro, Trinidad, from December 17, 2010 to January 6, 2011. Unfortunately, although the weather was good, if rather hot, it rained a lot during their last week. Ravi, Tara's uncle, said, "When I was young we had very little rain, but now we have a lot of rain." Ravi, a wealthy, good-looking man, lives in the north of the island.
I have spent most of the day putting in a comma and the rest of the day taking it out. Oscar Wilde