An apostrophe is used

1. To show ownership

a. Add 's when the owner word is singular or plural and does not end in s.
        Example: Your father's permission
The children's playroom
b. Add an apostrophe to only the last owner when more than one owner word is identified.
        Example:Tony, Hung and Maria's applications were lost.
c. Add the apostrophe to only the last word of a hyphenated word.
        Example: My sister-in-law's cat

2. To indicate missing letters in contractions

        Examples: it's (it is) and I'll (I will)

An apostrophe is not used with

1. Pronouns that show possession: hers, his, its, ours, theirs, yours
2. Plural nouns that do not show possession:
        Example: Two newspapers reported that the prices of new vans were the cheapest in six years.