The possessive case is used to express that something belongs to someone or to show a relationship between two or more people. We show this ownership by adding an apostrophe and an s to the noun if the noun is a person or an animal. We use the word "of" if there is a relation between two or more things.

With persons and animals:
Singular: 's Plural: s'
This is Tom's bike. These are the boys' bikes.
This is Kathy's mother. This is the girls' mum.
's with irregular plural forms: These are our children's cell phones.
's or s' with people's names ending in "s": This is Charles' / Charles's car.
To express an ownership for two or more people put the 's to the final name:
This girl is Ken and Bob's sister.
With things:  
The colour of the table is black. The second chapter of this book.


Exercise 1