We add 's to singular nouns to show possession:
We are having a party at John's house.
Michael drove his friend's car.
We add ' to plural nouns ending in -s:
This is my parents' house.
Those are ladies' shoes.
But we use 's with irregular plural nouns:
These are men's shoes.
Children's clothes are very expensive.
We can use a possessive instead of a full noun phrase to avoid repeating words:
Is that John's car?
No, it's Mary's. (NOT No, it's Mary's [
Whose coat is this?
It's my wife's. (NOT It's my wife's [