*The relation in which a noun stands to some other word or the change of form by which this relation is indicated, is called its case.—J. C. Nesfield.
There are five cases in English- Nominative, Objective, Possessive, Dative and Vocative. But in modern Grammar Dative is included in Objective. Hence there are four cases in English.
Nominative, Objective, Possessive and Vocative.
01. When a noun (or pronoun) is used as the Subject of a Verb, it is said to be in the Nominative Case. Example: John threw a stone. [Who threw a stone? = John (subject)]
02. When a noun (or pronoun) is used as the Object of a verb, it is said to be in the Objective case. Example: John threw a stone. The horse kicked the boy. [What did John throw? = a stone (object). Whom did the horse kick? =a boy. (Object)]
**Note: To find the Nominative put who? Or what? Before the verb. To find the Objective put whom? Or what? Before the verb. The Nominative generally comes before the verb and the Objective after it.
03. The Possessive case denotes ownership or possession or relationship or authorship. The possessive answers the question- 'whose?'
This is ram's umbrella. (possession)
These are Shakespeare's plays. (authorship)
A mother's love is a noble thing. (relationship)
Vocative case is practically a nominative of address. Examples: Come here, Ram. Come on, boys.
**Note: The forms of nouns remain the same in the Nominative case, Objective case and Vocative case. But the form is changed only in the Possessive case.
Formation of the Possessive case
The rules are as follows:
When the noun denotes the name of a living one, apostrophe s ('s) or apostrophe comma is used.
a) When such a noun is singular, the possessive case is formed by adding’s to the noun.
**Note: The latter s is omitted in a few words where too many hissing sounds would, come together. Examples: For justice’ sake; for goodness’ sake; for conscience’ sake moses’ laws; keates’ poem.
b) When the noun is plural and ends in s, the possessive case is formed by adding only an apostrophe. Examples: Boy’s club; school etc.
c) When the noun is plural but does not end in s, the possessive case is formed by adding ‘s. Examples: Men’s club; children’s books.
Use of possessive case
a) The possessive ('s) is chiefly used with the names of living things. It cannot be used with the names of inanimate things.
The boy's hand; the girl's hair.
The leg of the chair, [not, the chair's leg]
The cover of the book. [not, the book's cover]
The roof of the house. [not, the house's roof]
Of course, 'lf instead of’s may be used in both the cases: The leg of the boy. The leg of the chair.
b) The possessive (s') is used with the personified objects. Examples: Nature's laws; fortune's smile; duty's call.
c) The possessive ('s) is also used with the nouns denoting time space or weight. Examples: A day's match; a week's holiday; at a stone's throw; a foot's length; a pound's weight.
Noun in Apposition
*Apposition means placing near.—Wren & Martin.
A noun in opposition is in the same case as the noun which it explains.
01. Ram Chandra, the son of Disparate, went to forest.
02. Kavir, the great reformer, was a weaver.
03. Yesterday I met your father, the doctor.
04. Have you read the Gitanjali, the Tag ore's poems?
In sentences 1 and 2 the nouns in apposition are in the Nominative case.
In sentences 3 and 4 the nouns in appositions are in the Objective case.