A comma is used in the middle of, and never at the end of a sentence. This is done to make the sentence clearer, especially to separate items in a list. However, overusing commas can complicate a sentence, or render it meaningless. A comma is important to avoid any possibility of misunderstanding a sentence as the following example shows:

While my father was eating, my mother was drinking.
While my father was eating my mother was drinking.
Without the comma, it appears my father was cannibalizing my mother.

A comma is used:

• to separate words in a list

EXAMPLE: We saw monkeys, porcupines, lions and tigers in the zoo.

• to show a pause in a sentence.

EXAMPLE: Yesterday, as I was jogging in the park, I saw a black baboon.

• before inverted commas of actual words spoken.

EXAMPLE: He told us, "I believe the world is flat."

• to separate the name of the person spoken to.

EXAMPLE: "Jane, let me show you how to do it."

• to separate items in dates, that is day of week from the date, and day of month from the year.

EXAMPLE: He is marrying on Tuesday, 1st April.
EXAMPLE: The explosion happened on 1st January, 2000.

• to separate exclamations and expressions from the rest of the sentence.

EXAMPLE: Oh no, it can't be true.
EXAMPLE: Well, all I can say is it's a waste of money.

• after such words or phrases – of course, therefore, for example, however, moreover, etc. – when we begin a sentence.

EXAMPLE: Of course, you are always right.
EXAMPLE: Moreover, the rent is high and the location is not perfect.

• to separate parts of an address.

EXAMPLE: 999, Downing Street, Kingston.

• to separate cities and states/provinces, states/provinces and countries.

EXAMPLE: Montana, USA.

• to separate numbers that exceed three digits.

EXAMPLE: 2,000 / 20,000

• to separate greetings and closings in letters,

EXAMPLE: Dear Princess Diana, / Yours sincerely.

• to separate more than one adjective that modifies a noun.

EXAMPLE: The short, puny man was a comedian.

• to separate relative clause from the rest of the sentence.

EXAMPLE: The man, who lives next door, is a circus clown.

• to separate two independent clauses which together form a compound sentence.

EXAMPLE: Jack is washing the car, and Jill is washing the dishes.