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 required).
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
If you leave letters out of words, then show you have left them out by putting an apostrophe instead.
This is common with contractions:
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
Or some abbreviations:
The '60s were cool.
It is one o'clock (of the clock)
WARNING: In academic English, try to avoid the use of contracted (shortened) words. Use the full forms unless you are quoting someone.
Its or it’s?
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.
it’s = it is
its = of it
Dates and some abbreviations
These represent plural nouns which are not possessors and so never need apostrophes.
DVDs, CDs, MOTs