The subject of a sentence is a person or thing that performs the action of the verb. Subject pronouns are used to replace the subject (person or thing) of a verb. We do NOT normally say: 
John is tall and John is intelligent.
Saying the word "John" twice is repetitive and does not sound natural. We replace the Subject (John) that appears the second time with a subject pronoun to avoid repetition (and in this case to avoid saying the name John again.) So we would say: 
John is tall and he is intelligent.
We replace the second "John" with the Subject Pronoun "He".
What are the subject pronouns in English? These are the subject pronouns we use in English
Subject Pronoun
Singular or Plural - Who?
Masculine or Feminine
I
Singular - first person
masculine or feminine
You
Singular - second person
masculine or feminine
He
Singular - third person
only masculine
She
Singular - third person
only feminine
It
Singular - third person
object / thing / animal
We
Plural - first person
masculine or feminine
You
Plural - second person
masculine or feminine
They
Plural - third person
masculine or feminine
English Level I
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