The case of a noun (or pronoun) can dictate how it can be used in a phrase or clause. Three common cases of nouns are: Subject, Object, Possessive
Subject case: this is when the noun stands alone. For example, Mark is going to the store. Mark is the noun in subject case.
Object Case: this is when the noun is used for the object of a verb. For example, Mark is driving the red car. Car is noun in object case.
Possessive case: this is when any noun acts as an adjective, thus modifying another element in the same sentence. For example, the dog’s hair is wet after a shower. Dog is the noun with a possessive case.