Possessive 's

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:

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

one ball more than one ball
one boy
the boy's ball

the boy's balls
more than one boy
the boys' ball

the boys' balls

The structure can be used for a whole phrase:

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:

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

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

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 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 people's clothes