There are two basic reasons to use an apostrophe: contraction and possession.
Apostrophes are used as contractions (can’t, that’s, I’ve, etc…) and they are also used to show possession (ownership). For possession, to place the apostrophe in front of the “s” means that only one thing possesses it. Here are some examples with explanations:
- The girl’s stocking cap is knitted. (There is only one girl who has a stocking cap.)
- Look at that cow’s black spots! (We’re only looking at one cow in this case. It has black spots.)
To place an apostrophe behind the “s” means that more than one thing possesses it. (It can also be used for things which end in “s” or “z”, like Zeus’ lightning bolt.) Here are some examples of plural possessives:
- Those milkmaids’ aprons are red. (There are many milkmaids. Their aprons are red.)
- Those cows’ spots are funny-looking. (There are many cows. They all have funny-looking spots.)
And please note, who’s is always a contraction for who is. Whose is a pronoun representing a person to whom something belongs.