Apostrophes are one of the punctuation marks that many people seem to really struggle with. There are two main uses for apostrophes:

To indicate possession
To indicate omission

Possession

When we write about someone (the possessor) possessing something, then this is shown by an apostrophe. The rule is:

Put the apostrophe after the possessor (and add an ‘s’ if the sound requires it).

Examples:

One boy’s books - The books of one boy

Two boys’ books - The books of more than one boy

A woman’s rights - The rights of an individual woman

Women’s rights - The rights of (all) women

Toads’ fertility - The fertility of toads (in general)

The toad's fertility - The fertility of a specific toad

The USA’s voting record - The history of how the USA voted

Omission

If you leave letters out of words, then show you have left them out by putting an apostrophe instead.

Examples:

he is - he’s

will not - won’t

is not - isn’t

you are - you’re

There is - there's

I have - I've

Cannot - can't

WARNING: In academic English, try to avoid the use of contracted (shortened) words. Use the full forms unless you are quoting someone.

Common mistakes

Its or it’s?

it’s = it is
its = of it

This is the most common mistake people make about apostrophes – learn the rule, ‘it’s’ only needs an apostrophe if it means ‘it is’ not for the possessive.

Dates and some abbreviations

These represent plural nouns which are not possessors and so never need apostrophes.

Examples:

1960s

GPs, MPs

DVDs, CDs, MOTs