When we want to show that something belongs to somebody or something, we usually add an apostrophe + s ('s ) to a singular noun and an apostrophe (' ) to a plural noun, for example:

  • the boy's ball¬†(one boy)
  • the boys' ball¬†(two or more boys)

Notice that the number of balls does not matter. The structure is influenced by the possessor and not the possessed.

 one ballmore than one ball
one boythe boy's ballthe boy's balls
more than one boythe boys' ballthe boys' balls

The structure can be used for a whole phrase:

  • the man next door's¬†mother (the mother of¬†the man next door)
  • the Queen of England's¬†poodles (the poodles of¬†the Queen of England)
Although we can use of  to show possession, it is more usual to use possessive 's . The following phrases have the same meaning, but #2 is more usual and natural:
  1. the boyfriend of my sister
  2. my sister's boyfriend

Proper Nouns (Names)

We very often use possessive 's  with names:

  • This is Mary's car.
  • Where is Ram's telephone?
  • Who took Anthony's pen?
  • I like Tara's hair.

When a name ends in s , we usually treat it like any other singular noun, and add 's:

  • This is Charles's chair.

But it is possible (especially with older, classical names) to just add the apostrophe ':

  • Who was Jesus' father?

Irregular Plurals

Some nouns have irregular plural forms without s  (man → men). To show possession, we usually add 's  to the plural form of these nouns:

singular nounplural noun
my child's dogmy children's dog
the man's workthe men's work
the mouse's cagethe mice's cage
a person's clothespeople's clothes

sources : Original Link