The category of case

The category of case
The common case
The genitive case
is unmarked, it has no inflexion (zero inflexion)
is marked by the apostrophe s (‘s).

SINGULAR GENITIVE:
NOUN + ’ + S = A FRIEND’S FLAT
PLURAL GENITIVE (zero form):
NOUN + S + ’ = MY FRIENDS’ FLAT
Irregular plural nouns forming their plural by vowel
change also have the regular [z] in the genitive:
children’s games, women’s faces.
Nouns ending in –s form the genitive case in two ways:
Burns’ (Burns’s) poems, Dickens’ (Dickens’s) novels.

The genitive case is used:
1)
2)
3)
4)
With nouns denoting persons and animals:
John’s idea, the swallow’s nest, the mare’s back.
With nouns denoting time and distance:
a moment’s delay, an hour’s drive, today’s newspaper.
With the names of countries and towns:
Britain’s national museums, Canada’s population.
With the names of newspapers and nouns denoting
different kinds of organizations:
the Guardian’s analysis, the Tribune’s role,
the company’s plans,

5. Often with the nouns world, nation, country, city, town:
the world’s top guitarists, the nation’s wealth.
6. With the nouns ship, boat, car:
the ship’s crew, the car’s wheel.
7. With nouns denoting planets: sun, moon, earth:
the sun’s rays, this earth’s life.
8. With some inanimate nouns in the following set expressions:
to one’s heart’s content (desire), at death’s door, at arm’s length, out of harm’s way, a hair’s breadth, a needle’s eye, at a stone’s throw, to move at a snail’s pace, at the water’s edge.

Group genitive when ‘s can be joined:
4) to a group ending in a numeral:
in an hour or two’s time.
3) to a noun (pronoun) + a pronoun group:
someone else’s benefit.
2) to a more extensive phrase which may even contain a clause:
the Duke of Norfolk’s sister, the secretary of state’s private room,
the man I saw yesterday’s son.
to a group of two coordinated nouns if such a group refers to a single idea:
Mum and Dad’s room, John and Mary’s car.

Absolute genitive =
NOUN + ’ + S – HEADWORD
To avoid repetition:Our house is better than Mary’s (than Mary’s house).
After the preposition of: an old friend of my mother’s, that cousin of my husband’s.
To denote shops such as the butcher’s, the baker’s, the grocer’s, the chemist’s, or institutions, where the genitive is usually a saint's name: St Paul’s (Cathedral),
St James’s (Palace), or places of residence: at Timothy’s, at Old Jolyon’s, at my uncle’s.