Possessive forms of Nouns: There is a difference between the plural forms of the nouns and the possessive forms. When we use the plural form of the noun, we just mostly add 's' with the noun. Example:boys,teachers.
However, when we want to say something belongs to someone or something is associated with someone, we use an apostrophe plus 's' to the nouns.
Here are some examples:
The girl's eyes are blue. (singular noun)
The boys' hostel is located close to their college. (plural noun)
The children's park is very interesting. (irregular plural not ending with 's')
Raj's school bag is very heavy. (proper noun)
Charles'mother is calling him. (Proper noun ending with 's')
We also use 's with words or expression signifying time:
Example: an hour's drive, today's programme.
The bag's zip is torn.
The students' library has a good collection of books.
Her parents' behaviour is very polite.
Please hand this parcel over to the lady at the women's hostel.
Rani's father is a doctor.
The project will take a month's time to complete.