The Magical Apostrophe S
Contractions always take apostrophe s
The apostrophe in contractions takes the place of one or more letters. It's a rule. Why? Because it is a rule. The apostrophe in it's takes the place of the i in is.
Plurals never never have apostrophe s (almost).
A bunch of bears went to their cave, not a bunch of bear's.
RIGHT: I own several TGPs. WRONG: I own several TGP's.
The exception: individual letters, numbers, punctuation, etc do take apostrophe s. So the word banana has 3 a's, not 3 as.
Possessives always always have apostrophe s (almost).
Nancy's pet. Her cat's toy.
The exception: possessive pronouns (its, his, her) do not take apostrophe s.
So it's (it is) his book, and the cover is its cover, not it's cover. That book is hers (not her's).
What if it's plural and possessive? Follow these examples:
"My employee
s' cafeteria."
"My kids' bedroom."
Need practice?
http://www.better-english.com/grammar/aps.htm