Learn the English subject pronouns and how to form basic "Subject - Verb" sentences.
Pronouns, basic feelings, introductory greetings
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.
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).
Subject pronouns that rename the subject in the predicate occur after the verb "to be."
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, the "I" comes after "is" because that is the conjugated form of "to be" for "it"; however, "am" occurs after "who" because verbs after that particular pronoun must match the pronoun ("I") being referred to.