From my 11-24-07 entry:
Question: When do you use an apostrophe?
apostrophe: The apostrophe has three uses:
1) to form possessives of nouns
2) to show the omission of letters
3) to indicate certain plurals of lowercase letters.
Apostrophes are NOT used for possessive pronouns or for noun plurals, including acronyms.
How to make a noun possessive: To see if you need to make a possessive, turn the phrase around and make it an "of the..." phrase. For example:
the boy's hat = the hat of the boy; three days' journey = journey of three days
Once you've determined whether you need to make a possessive, follow these rules to create one.
--add 's to the singular form of the word (even if it ends in -s): the owner's car, James's hat
--add 's to the plural forms that do not end in -s: the children's game, the geese's honking
--add ' to the end of plural nouns that end in -s: houses' roofs, three friends' letters
--add 's to the end of compound words: my brother-in-law's money
--add 's to the last noun to show joint possession of an object: Todd and Anne's apartment
Don't use apostrophes for possessive pronouns or for noun plurals. Apostrophes should not be used with possessive pronouns because possessive pronouns already show possession -- they don't need an apostrophe. His, her, its, my, yours, ours are all possessive pronouns.
This is one of my buddy Greg's (note the apostrophe there, denoting possession) pet peeves, and it has also become one of mine. Specifically, "it's" means "it is." It is (or I could write "it's") not interchangeable with the possessive "its," as in "a leopard cannot change its spots." If you substitute "it's" in that sentence, the expanded contraction would read, "a leopard cannot change it is spots." It makes no sense.