When To Use “'s” At The Ending Of The Word? [duplicate]

Question

Possible Duplicate:
What is the correct possessive for nouns ending in s?

First, sorry about my English.

I would love to know when I need to add 's at the ending of the word.

1) When I meant is, like...

He's famous.

...to:

he is famous.

2) When it's like possession (don't know is this right word for that) of that word? And... exactly when??

John's book.

Dream's city.

Alien's ship. (one alien)

Aliens's ship. (two or more aliens)

Answer 1

I'll paste an answer I wrote some time ago.

It's called "Saxon genitive"; the general rule is that you use it after people/animals/things and never with adjectives or with personal pronouns (you'd never say "this is he's car", since "he's" always means "he is".

In this case you would use the possessive pronouns his, her, theirs, etc.

Plus, if the noun is plural you can say "Aliens' ship".