To be

I am = I'm

I'm a student.

I'm not a student

He is a student / Tom is a student / My friend is a student

Tom is a student.

Is Tom a student? - Yes, he is

Tom is not (isn't) a student

They are students / Maria and Sarah are students

They are students.

Are they students? - Yes, they are

Maria and Sarah are not (aren't) students

You are a student

Are you a student? - Yes, I am

Are you a teacher? - No, I am not

You are not (aren't) a teacher