What is a subjective case personal pronoun?

The subjective case pronouns function as the subject of a sentence or a clause.

The subjective personal pronouns are: I, you, he, she, it, we, they.

Examples:

I saw this movie before. (subject of the sentence)

The train that he took was late. (subject of the relative clause)

Jane, you are a good friend. (subject of the sentence)

She made the kind of cookies we like. (subject of the dependent clause)

John finished his homework and he went to bed. (subject of the second half of the compound sentence)