The possessive form is used to indicate ownership.
Possessives in English are formed according to a set of rules. Here is a brief summary:

RuleExample
Singular Nouns :
(even if the meaning is plural
or if there are several words.)
Add ‘s after the possessor.– Tom’s car
– The cat’s tail.
– The children’s toys
– The Queen of England’s jewels
Plural NounsAdd ‘s after the plural ‘s’.– My parents’ car.
– The students’ rooms.
– My grandparents’ house
Names/nouns ending in ‘s’Add ‘s after the last ‘s’.– Denis’s wife
– Gladys’s job
– The boss’s office.
Double PossessiveAdd ‘s after the possessor’s name,
or use a possessive pronoun.
– A friend of my father’s
(one of his friends, not the only one)
– A friend of Julie’s
– A friend of
mine/his/hers/ours/yours/theirs.
Shops and people’s homesOften take the possessive.– At the hairdresser’s
– At the dentist’s
– At the Brown’s

N.B.
The apostrophe followed by s (‘s) (ex: Tom’s), to denote possession, is used for living things.

  • living things (humans and animals) :
    • John’s car, the cat’s milk, the dog’s tail (not the car’s door)
  • groups and institutions :
    • The government’s proposal, the company’s policy.