Noun Case:

When a noun (or pronoun) is used as the Subject of a verb, it is said to be in the Nominative Case.

When a noun (or pronoun) is used as the Object of a verb, it is said to be in the Objective Case (or Accusative Case )


Examine the sentence :-

This is Ram's Umbrella
Ram's Umbrella = the Umbrella belonging to Ram.
The form of noun Ram is changed to Ram's to show ownership or possession.
The noun Ram's is therefore said to be in the Possessive (or Genitive) Case.