The possessive case indicates possession.
Read the sentence given below.

This is John’s car.

Here the noun John’s is in the possessive case. A noun which shows ownership or possession is said to be in the possessive case.

How the possessive case is formed

When the noun is singular, we make the possessive form by adding ’s to the noun.

John’s father
Mary’s goat
Susan’s car

Shyam’s daughter

When the noun is plural and does not end in -s, the possessive case is formed by adding ’s to the noun.

Men’s hostel
The People’s Republic
Women’s liberation
Children’s books

When the plural noun ends in –s, we simply add an apostrophe (’) without –s to form the possessive.

A boys’ school
Girls’ hostel
Students’ contribution
Parents’ rights

Singular nouns ending in –s may form the possessive by adding an apostrophe (’) with or without –s.

Thomas’s house OR Thomas’ house
Yeats’s poems OR Yeats’ poems

In compound nouns, ’s is added to the last word.

My sister-in-law’s daughter

Notes

The letter –s is omitted and only the apostrophe (’) is added when too many hissing sounds occur in a word.

For conscience’ sake
For goodness’ sake
For Jesus’ sake