Apostrophes have two uses:
Apostrophes show you that some letters have been taken out of a word to shorten it. For example:
Do not becomes don’t. I will becomes I’ll. Could have becomes could’ve.
The apostrophe goes where the letters have been removed. Apostrophes are used this way in informal writing. You should not shorten words when you are writing formal letters.
NOTE: sometimes words are shortened in an irregular way. The apostrophe, however, is still used to show where letters are missing. For example:
Will not becomes won’t.
Apostrophes show that something belongs to or is connected with something else. To show belonging you add: ’s
The cat’s tail - says that the tail belongs to the cat. The newspaper's readers - says that the readers are connected with the newspaper. Tony’s hair - says that the hair belongs to Tony.
NOTE: usually the apostrophe goes before the ‘s’. If the owner already ends in ‘s’ then the apostrophe goes after the ‘s’ that is already there. You just need to add an apostrophe. For example:
The dogs’ bowls - says that the bowls belong to some dogs. The boys’ coats - says that the coats belong to some boys. The cars’ wheels - says that the wheels belong to some cars.
Watch out for plurals that don’t end in ‘s’. Words like ‘men’ and ‘children’ don’t end in ‘s’, but they are talking about lots of people. These words use ’s to show possession. For example:
The men’s hats - says that the hats belong to the men. The women’s house - says that the house belongs to the women.
It’s and Its
It’s means ‘it is’: eg It’s cold outside today. But there is no apostrophe after ‘it’ to show belonging: eg The kitten likes to chase its own tail.