1)

to separate independent clauses

The following conjunctions are used in such cases:
and, but, for, nor, or, so & yet¹

The workers demanded extra pay, but the management refused to give it.

2)

after introductory

a) sub-clauses
b) phrases
c) words

a) If you don't finish it tonight, you'll be late.
b) As a matter of fact, I'm going on vacation to Singapore next week.
c) Unfortunately, we aren't able to agree to all your demands.

3)

when

a) sub-clauses
b) phrases
c) words

appear in the middle of the sentence.

a) Jill, who was sitting behind her desk, gave Tim a smile.
b) We, as a matter of course, will contact your former employer.
c) We have, however, found a number of errors.

4)

to set off three or more words, phrases or main clauses in a series.

She went into the office, sat down at her desk, and started surfing the Net.
He's lived in London, Paris, Hong Kong, Tokyo and New York.

5)

to set off two or more coordinate adjectives if the meaning does not change when the order is altered.

We had to travel over several narrow, winding, dangerous roads.

6)

at the end of a sentence in order to indicate a pause.

He was just ignorant, not stupid.

7)

to set off a nonrestrictive (also non-defining) relative clause².

Unitech, which was established in 1992, employs over 750 workers.

8)

when someone is addressed directly.

Richard, can you do me a big favour?

9)

to show an appositive³.

Chris Patton, former governor of Hong Kong, is still very popular in Hong Kong.

10)

in dates.

Yes, May 11, 20xx, was the date of the last AGM.

11)

in front of tag questions.

You've met this client before, haven't you?

12)

after digits indicating thousands.

10,000
9,999,999