A subject pronoun is used as the subject of a verb, which means that it’s the person or thing performing the action. It usually appears before the verb, at the start of a sentence (e.g., “He ran home”).
The subject forms of the personal pronouns are I, we, you, he, she, it, and they. The subject form of the interrogative pronoun or relative pronoun used to refer to people is who.
All other pronouns (e.g., “this,” “somebody,” “many”) have only one form that is used for both subject and object.