Use of Verb to be

Use of be verbs as an auxiliary verb

Form
We know that be verbs are am, is, are, was, were, 

Principal parts: be, was, been
Gerund/present participle: being
For properly understanding  sentence with structures are given below.

***Verb to be ( am, is, are) these  are used in present form.
***Am is used with 1^st person singular number. 

Affirmative form
***Subj.+am+extension

Example: I am a teacher.  (If person is something/someone)
I am here today.  (If person is somewhere)
I am in the room.
**Is is used with 3^rd person singular number.

Sub+is+extension.
Example:
He  is my friend. (If person is something/someone)
She is a doctor.
Shimu is a good singer.
Her father is a rich man.

He is in the class. (If person is somewhere)
**Are is used with 2^nd person singular or with all sorts of plural numbers.
Sub+are+extension.

Example: You are a student. (If person is something/someone)
They are my friends.
We are your neighbors.
You are good players.
We are here in the room. (If person is somewhere)

Negative form
Subj.+am/is/are+not+extension.

Example:- I am not a farmer.
He is not a student.
You are not a poor man.
He is not a good man.
You are a great liar.

Interrogative sentence
Am/is/are+subj.+extension?

Am I your friend?
Is he your brother?
Are you ill today?
How are you?
Who is she?
What is your name?
What is your father?
How is your mother?
Who is this man?

**Was and were are used in the past form
**Was is used with 1^st person and 3^rd person singular number.

Affirmative form
Sub+was+extension.

Example: I was a student. (If person is something/someone)
He was a police officer.
She was a house wife.
He was in the field. (If person is somewhere)

**Were is used with 2^nd person singular number and all sorts of plural numbers.
Subj.+were+extension

You were there at that time. (If person is something/someone)
We were present here yesterday.
They were the students of this school.
We were in the market. (If person is somewhere)
They were here ten minutes ago.

Negative form
Subj+was/were+not+extension.

 I was not there at that instant.
You were not here in the morning.

Interrogative sentence
Was/were+subj+extension?

Were you here then?
Who was here?
Where were you so long?
Where was he yesterday?
How were you in America?