Plural possessives and singular possessives

7 years ago

How do I know when to use the apostrophe?
If the noun is singular add 's
The girl's gift 
If the noun is plural 
(2 girls have a gift)
The girls' gift
If a singular noun already ends in s like Walrus or Chris
If it's a common noun ad 's The walrus's tusk.
If it's a proper noun add ' only Chris' tooth.