Case is a grammatical category which shows relation of the noun with other words in a sentence. It is expressed by the form of the noun.
English nouns have 2 cases: the common case and the genitive case. However, not all English nouns possess the category of case; there are certain nouns, mainly nouns denoting inanimate objects, which cannot be used in the genitive case.
The common caseis unmarked, it has no inflexion (zero inflexion) and its meaning is very general.
The genitive caseis marked by the apostrophe s (’s).
In writing there are 2 forms of the genitive: for most nouns it is ’s (mother’s) and for nouns ending in -sand regular plural nouns only the apostrophe (mothers’). Irregular plural nouns also add ’s to the noun (women’s, children’s).
Besides nouns denoting living beings we can use ’swhen a first noun is
1. an organization (= a group of people), ships, boats, names of people to mean a house:
the government’s decision, the company’s success, the ship’s captain. We met at Bill’s.
2. the place: town, city, country, ocean, world, the sun, the moon, the earth, names of the countries: the city’s new centre, the world’s population, Britain’s government, Russia’s exports.
3. time & distance: an hour’s rest, two hours’ drive, month’s holiday, night’s sleep, in two years’ time, ten minutes’ break=a ten-minute break.
4. time words: tomorrow’s meeting, today’s paper, Sunday’s dinner.
5. names of newspapers: The Guardian’s analysis, The Tribune’s role.
When two persons possess or are elated to something they have in common: Mum & Dad’s room, John & Mary’s car.
In compounds and names consisting of several words the last word takes apostrophe ’s: My sister-in-law’s guitar. My father-in-law’s son. Henry the Eighth’s wives. The Prince of Wale’s helicopter.
Sometimes certain nouns can be used in the possessive case without the second noun: the baker’s, the chemist’s. It means the baker’s shop, the chemist’s shop.
It can also be used after the initials & the names of the owners of some business: the PM’s secretary, the MP’s briefcase, Selfridge’s, Sotheby’s, but Harrods, Foyles.
With Greek nouns in –sof more than one syllable: Socrates’, Euripides’.
After the preposition of: an old friend of my mother’s, that cousin of my husband’s.
With for + noun+ sake: for heaven’s sake, for God’s sake.
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
Of+ nounis used :
1. When the possessor noun is followed by a phrase or a clause: I took the advice of a couple I met on the train & hired a car.
2. With inanimate possessors: the walls of the town, the key of the car.
3. With the words denoting quantity: part, slice, bit, etc.: a piece of cake, a slice of bread, a pound of butter, a bit of news.