Comma

Comma

is used to separate things which are to be mentioned one by one.

Examples:
The assignment requires us to think, write and speak.
Ali is a man of courage, strength and strong will power.
For the purpose of research the scientist traveled to China, Mexico, Brussels, Santo Domingo and Puerto Rico.

Use of Comma before the conjunction and

A comma is normally not used before the last element in a series, that is, before the conjunction and. An exception is made, however, if the sentence would otherwise be ambiguous (i.e it may shoe more than one meanings):
Example:
The months with the most connections are October and November, and March, April and May, coinciding with the start of the semesters.

Before a coordinating conjunction

A comma is used before a coordinated conjunction. Below are few examples. Example:
Our son is having an award next week, and we are going to celebrate it.
I have very few friends, but all are much loyal.
The exception is that we can omit comma if the coordinated sentences are too short.

In parenthetical expressions:

A comma is used in parenthetical expressions as in the below example. Examples:
Zaighum, noticeably excited, told the news.
Zahra, my class mate, commented about my personality.

Elisions

In sentences where two statements are joined or merged.
Examples:
Italy is famous for her composers and musicians, France, for her chefs.
Pakistan is a beautiful country; and it's capital, a great city.

In addressing

When addressing another person.
Example:
Hey, Ali, where are you going?

In interjections and asides

Examples:
Good grief, what a mess!
Your comments, if you don’t mind my saying so, reveal an astonishing ignorance of the situation.

To separate digits

We use commas to separate digits.

Example:
89,000 Rupees.