Possessive's

When we want to show that something belongs to somebody or something, we usually add '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.

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:

the boyfriend of my sister
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 'sto the plural form of these nouns:

singular noun	plural noun
my child's dog	my children's dog
the man's work	the men's work
the mouse's cage	the mice's cage
a person's clothes	the people's clothes