The subject in a sentence is the entity (person, thing, object or place) that is doing something. When a pronoun (I, you, he, she, it, we, you, they) takes the place of a subject in a sentence, it becomes a subject pronoun.
Examples:
I (subject) am driving a car (object).
I (subject) am calling him (object pronoun).
We (subject) are going to see them (object pronoun).
She (subject) will message you (object pronoun).
Other pronouns can also be subjects, for example: who, whoever, everybody, everyone, many, some, someone, and somebody. This is not a complete list, but covers the most common ones.
In very formal English, subject pronouns that rename the subject in the predicate occur after the verb “to be.” In everyday English, people usually say “This is me” or “It’s me.”
Examples:
Phone caller: Hello. Can I please speak with Anne?
Anne: This is she.
It is I who am sorry for the mess.
Note that in the first example, “this” is a stand-in for “she” (Anne). In the second example, “I” comes after “is” (the form of “to be” that agrees with “it”); however, “am” comes after “who” because the verb after “who” must agree with the pronoun it refers to (“I”).