Example:

That man is back.

They're back.

He's back.

They're back.

They have come back. They're back.

Take nouns, adjectives, adverbs of place or phrases as complements.

This man is a teacher. ?

The teacher is a supplement, a supplementary explanation to people.

Is is a component that connects the subject and the complement.

Mary's new clothes are brightly colored. Mary's new skirt is brightly colored.

Same as above. Colorful is a complement.

My mother is in the kitchen. My mother is in the kitchen.

They are teachers. They are teachers.

He used to be a teacher. He used to be a teacher.

He has been a teacher for three years. He has been a teacher for three years.

Add not after am, is, are, was and was.

The abbreviation is not, not, not, not, not, not, not, not.

Am, is, the present tense, was, the past tense.

The man didn't come back. The man didn't come back.

I did not come back. I did not come back.

They didn't return. They didn't return.

The above is the present tense

He didn't come back. He didn't come back.

They didn't return. They didn't return.

The above is the past tense

The be verb becomes a question, the be verb is placed at the beginning of the sentence, and other components remain unchanged.

Is he a teacher? Yes, he is. No, he isn't.

Is he a teacher? Yes, he is. No, he isn't.

Are you a teacher? Yes, I am. /No, I'm not.

Are you a teacher? Yes, I am. /No, I'm not.

Are they teachers? Yes, they are. No, they aren't.

Were they teachers before? Yes, they are. No, they're not.

Exercise:

Is he a doctor? Is he a doctor?

No, he isn't. No, he isn't.

Were they in the classroom yesterday? Were they in the classroom yesterday?

Yes, they are. Yes, they are.

They were not in the classroom yesterday.

Answer: They were not in the classroom yesterday.