Forming the Possessive Case
The Possessive case (or as it's also known, Genitive case), is that form in which a noun is used in order to indicate that something belongs to the person or thing for which it stands.
In English the possessive case is not only used to indicate belongings or possessions but it’s also used in other ways to indicate various relations between the noun marked for possession and the noun that follows:
|daddy's pet, the president's adviser||possession or belonging|
|the tide's ebbing, Army's presentation||subject of action|
|the company's lawyer, the hero's betrayal||object of an action|
|learner's dictionary, a women's college||description or type|
|Gretchen's kindness, the hero's bravery||attribute|
|Beethoven's symphonies, grandmother's letter||origin|
|a day's journey, an arm's length||measurement, amount|
In the sentence “I borrowed Gretchen’s piano,” the noun Gretchen’s is in the possessive case, to indicate that Gretchen possesses something (namely, a piano). Therefore, Gretchen’s, is the possessive case of the noun Gretchen.
The noun in the possessive case is said to be governed by the noun which stands for the property which is possessed. In the example above, the noun Gretchen’s is governed by the noun piano.
As what is possessed must be a person or a thing of some kind, a noun in the possessive case can only be governed by a noun.
The meaning of the possessive case is sometimes expressed by means of the preposition of, with the objective case after it. For example, for “Gretchen’s book,” we may say, “The book of Gretchen.” However, the possessive case must not be substituted for the preposition of, unless possession is implied by it.