The possessive forms of nouns are formed by adding  ’s to the noun.

Singular noun + ’s (John’s car, father’s house etc.)
Plural noun + ’ (my parents’ house)
Irregular plural + ’s (men’s hostel, children’s books)

We sometimes just add an apostrophe (’) to a singular noun ending in –s.

Socrates’ philosophy

But ’s is more common

Denis’s car
Tess’s boyfriend
Dickens’s novels

Pronunciation

The ending ’s is pronounced just like the Plural ending. Note that the apostrophe in words ending –s (e.g. parents’) does not change the pronunciation at all.

Possessives and other determiners

A noun cannot have an article and a possessive word with it.

Peter’s dad (NOT the Peter’s dad) (NOT Peter’s the dad)

But note that a possessive word can have its own article.

The boy’s cap

When we want to use a noun with an article or a demonstrative and a possessive word we use a structure with of.

She is a cousin of Peter’s. (NOT She is a Peter’s cousin.)
Take that dirty fingers of yours off me. (NOT Take that your dirty fingers off me.)

We can use a possessive word without a following noun, if the meaning is clear from the context.

‘Whose is this?’ ‘Mary’s.’

In modern English, expressions like the doctor, the dentist, the hairdresser, the butcher etc are often used without ’s.

She is always at the hairdresser. (= She is always at the hairdresser’s.)