One step at a time
Past Tense

Subject + To Be(past) + Not + Complement
I           was       not     a  student
You     were     not     a  student
He       was      not     a  student
She     was      not     a  student
It         was      not     a  desk
We     were    not      students
You    were   not       students
They  were   not       students
Past Tense
Subject +( To Be(past) + Not) + Complement
I           wasn't      a  student
You     weren't    a  student
He       wasn't     a  student
She     wasn't     a  student
It         wasn't     a  desk
We     weren't   students
You    weren't  students
They  weren't   students