Can a subject pronoun be used as a subject?

Yes, the subject pronouns function as the subject of a sentence or a clause.

The subject pronouns are: I, you, we, he, she, it, and who.

Examples:

I do like that movie. (subject of the sentence)

The movie that I like is on TV tonight. (subject of the relative clause)

You may like the movie too. (subject of the sentence)

You will also like the actor who is in it. (subject of the relative clause)