In English, it has two jobs:
- To show where one or more letters have been left out (as in the abbreviation (contraction) of do not to don't).
- To show the possessive case (as in the cat’s whiskers).
Its versus it's[change]
- The cat chased its tail. Correct.
- The tyre lost it's grip. Wrong.
It's: a contraction of the verbal phrase It is or It has.
- It's mine. Correct; check: It is mine.
- It's been here. Correct; check: It has been here.
- The cat chased it's tail. Wrong; cannot be expanded to It is.
The same applies to yours, theirs and ours because these are also possessive adjectives of personal pronouns.
- The colour is ours.
- That book is hers (or his).
- Theirs was the responsibility.
Apostropes are also used to show something belongs to someone (or something). Again, correct uses can be expanded:
- Mike's car. Correct: the car that belongs to Mike.
- The dog's ball. Correct: the ball that belongs to the dog.
- Those dog's are large. Wrong: cannot be expanded. Here "dogs" is a plural word.
The intrusive apostrophe[change]
- Mrs. Jones' hat or Mrs. Jones's hat. Both correct.
- Both of my parents' birthdays. Correct.
- CD's and DVD's: wrong
- Apple's and pear's: wrong
Writing dialogue or titles[change]
Apostrophes are also used when other words are shortened, as in slang:
- Go get 'em tiger! or Li'l Bow Bow.
This is just a version of the abbreviation function.
- Quirk, Geenbaum, Leech & Svartvik, A comprehensive grammar of the English language. Longman, London & New York. p985 ISBN 0-582-51734-6