Apostrophes are all about ownership. If you want to illustrate this, work out who the owner is, then add an apostrophe and an “s”. For instance:
- the owner of a MacBook is the MacBook's owner
- a girl's book
- the child's book
- the children's books
The way to test if you’ve got the apostrophe in the right place is to cover up the apostrophe and everything after it. Is the word that is left the owner? If not, your apostrophe is in the wrong place.
With plural words, the same rules apply, but you may omit the extra 's' after the apostrophe.
- the kids’ book
- the readers’ tutorial
Some tricky ones to remember:
- I am going to read the tutorial in one hour’s time
- I am going to read the tutorial in two hours’ time
In this case, the hours belong to the noun, time.
Words such as his, hers and its do not require apostrophes because ownership is already built into these words.