You can use an apostrophe to show that something belongs to someone (or sometimes something) e.g.

Jack’s car is parked over there.
The city’s police department ranked 1st in the nation.
Rosetta’s little sister works as a secretary.
The people’s lives are in danger.

Note: If the name ends in “s”, you must put the apostrophe after “s” e.g.

Charles’ new shop is about to open soon.
This must be Francois’.

Note: If the word is plural (ending in “s” or “es”, again the apostrophe comes after the “s” e.g.

These cars’ wheels need replacement.

You can also use apostrophe in contracted forms when you omit some letters to make the word shorter. For example:

They don’t really mean anything.
He’d rather join the club.
It’s necessary to reduce the level of carbon dioxide in cities.

Note: Do not confuse “it’s” with “its”:

It’s part of the job. (It is part of the job.)
Its part is broken. (The part belonging to it is broken.)

When you want to pluralize one letter or a number. For example:

He passed his exam with three A’s.
Your v’s are written like r’s.
Did you get all the 10’s?

Note: This rule does not apply when there is a group of letters.

There were more than 100 MPs in the parliament today.

A Note to IELTS candidates