What is a pronoun is a nominative case?

The term 'nominative case' is another term for 'subjective case', a noun that acts as the subject of a sentence or clause.

The nominative case pronouns are: I, you, he, she, it, we, they, who.

Examples:

When George got to 19th Street, he got off the train.

The children were so pleased with the cookies that they made.

You should ask the teacher who assigned the work.

A nominative case pronoun also functions as a predicate nominative (also called a subject complement).

A predicate nominative is the noun or a pronoun following a linking verb that restates or stands for the subject.

Example: The winner of the trophy was you. (winner = you)