An apostrophe can be used to mark missing letters where two or more words have
been 'contracted', for example
do not becomes
apostrophes are used to show where one thing or group
belongs to another. Where the thing or group belongs to a singular noun or
an irregular plural, the apostrophe goes before the s. For example, Jack's
tie, the car's wheel, Mr. Smith's house.
Where the group or thing belongs to a regular plural (ending
in s), the apostrophe goes after the s. For example, the girls' books (books
belonging to a group of girls), the cows' field (the field belonging to a
group of cows).