Cases describe the grammatical functions of nouns, pronouns and noun phrases, such as whether they are the subject of the clause or a subordinate object.
The nominative case, also called the subjective, marks the subject of a phrase. The subject is the noun (or pronoun) that performs the action of the verb. For example, in the phrase "She helped him", the subject is "she", who performs the action of helping.
The oblique case is used to mark nouns and pronouns as being direct and indirect objects. For example, in the phrase "I gave it to him" both "it" and "him" and objects, and thus are in the oblique case.