comma is used in the middle of, and never at the end of a sentence. This is done to make the sentence is complete, and also 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:

Examples:

  • While my father was eating my mother was drinking.

    (Without the comma, it appears my father was eating my mother.)

  • While my father was eating, my mother was drinking.

 

A comma is used to separate:

Examples:

words in a list.

  • We saw monkeys, porcupines, lions, and tigers in the zoo.
  • I met her parents, uncles, aunties, cousins, brothers, sisters, and pet dog.

more than one adjective that modifies a noun.

  • The short, puny man was a comedian. 

names and titles.

  • Mr. Hillyholly, the General Manager.

 

A comma is used to separate:

Examples:

parts of an address. 

  • 999, Downing Street, Kingston

cities and states / provinces, states / provinces and countries

  • Montana, USA

items in dates, that is day of week from the date, and day of month from the year

  • He is marrying on Tuesday, 1st April.
  • The explosion happened on 1st January, 2000.
  • It happened on September 11, 2001.

numbers that exceed three digits

  • 2,000 / 20,000 / 200,000 / 2,000,000

 

A comma is used after:

Examples:

the name of the person spoken to or directly addressed.

  • Jane, let me show you how to do it.

exclamations and expresions from the rest of the sentence.

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

greetings and closings in a formal letter

  • Dear Princess Diana, / Yours sincerely / Yours truly

such words or phrases: of course, therefore, for exmaple, however, moreover, etc. to begin a sentence.

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

a phrase that begins a sentence.

  • From the tunnel, it took the train another hour to reach our destination. (Prepositional phrase)
  • Attracted by the brightly-coloured flowers on sale, we decided to have a closer look. (Paricipial phrase)

a subordinate clause.

  • When they saw me, they pretended they didn't know me.

 

A comma is used to set off interrupting words, which can be: 

Examples:

an expression.

  • The cheetah, don't you know, is the fastest animal in the world.

a clause. 

  • The man, who lives next door, is a circus clown. (Relative clause)
  • Yesterday as I was jogging in the park, I saw what looked like a black baboon. (Subordinate clause)

a nonrestrictive clause. 

  • Traditional Indian dishes, which are my all-time favourites, are available in this restaurant.

an appositive.

  • Her sister, a well-known eye surgeon, was admitted for eye surgery.
  • An insomniac, my grandfather goes to bed when others get up to go to work.

a direct quotation.

  • He said, "I believe the world is flat."
  • "This town's history," he said, "goes back to Roman times."

 

A comma in a compound sentence
A comma is used where there are two independent clauses joined by a coordinating conjunction. The two clauses together form a compound sentence. The comma is used before the coordinating conjunction.

 

Examples:

  • Jack is washing the car, and Jill is washing the dishes.
  • He doesn’t know how to swim, but his wife does.