The possessive is the only case of nouns that is indicated by a change in form.
A noun or pronoun used as an attribute complement is in the nominative case. A noun or pronoun following a preposition as the principal word of a phrase is in the objective case. I and he are nominative forms. Me and him are objective forms.
The following sentences are therefore incorrect: It is me; It is him; Me gave the pen to he.
+DEFINITIONS.
Person is that modification of a noun or pronoun which denotes the speaker, the one spoken to, or the one spoken of.
The First Person denotes the one speaking.
The Second Person denotes the one spoken to.
The Third Person denotes the one spoken of.
Case is that modification of a noun or pronoun which denotes its office in the sentence.
The Nominative Case of a noun or pronoun denotes its office as subject or as attribute complement.