Case indicates the relations of the noun (or pronoun) to the other words in the sentence.
English nouns denoting living beings (and some nouns denoting lifeless things) have two cases, an uninflected form called the common caseand an inflected form called the genitive case.
1. The genitive caseis formed by adding -’s (the apostrophe s) to the noun in the singular and only ’ (the apostrophe) to plural forms ending in ‑s.
SINGULAR: a girl’s book PLURAL: a girls’ school
N o t e 1. — Nouns forming their plural by changing the root vowel take the
apostrophe s in the plural.
SINGULAR: a man’s hat PLURAL: men’s hats
N o t e 2. — Nouns ending in ‑s form the genitive case in two ways: Dickens’
novels, Dickens’s novels.
The pronunciation of the genitive case ending follows the same rules as the pronunciation of the plural ending:
[Iz] after sibilants: prince’s, judge’s, witch’s, etc.
[z] after voiced consonants other than sibilants and after vowels: boy’s, man’s, king’s.
[s] after voiceless consonants other than sibilants: Smith’s, count’s, bishop’s.
N o t e. — With nouns ending in ‑s and forming the genitive case in two ways
(Dickens’ novels, Dickens’s novels) the ending is pronounced [Iz] whether the
letter s is written or not.
2. Sometimes the apostrophe s may refer to a whole group of words (the group-genitive): Jane and Mary’sroom. The last word of the group need not even be a noun: I shall be back in an houror two’stime.
As to its use the genitive case falls under:
(A) The Dependent Genitive.
(B) The Absolute Genitive.
The Dependent Genitive is used with the noun it modifies and comes before it.
The Absolute Genitive may be used without any noun or be separated from the noun it modifies.