One step at a time
Present Tense

Subject + To Be + not + Complement
I             am     not    a  student
You       are     not    a  student
He         is       not     a  student
She       is       not     a  student
It           is       not     a  desk
We       are    not    students
You      are    not    students
They    are    not    students
Present Tense
Negative (Contraction)
I'm            not      a  student
You're      not      a  student
He's          not      a  student
She's        not      a  student
It's            not      a desk
We're       not      students
You're      not      students
They're    not      students
Negative (Contraction)
I'm           not        a  student
You         aren't    a  student
He           isn't       a  student
She         isn't       a  student
It             isn't       a desk
We         aren't   students
You        aren't  students
They      aren't  students