Saying ‘Yes, I do. / No, I don’t’ in English is more polite than just saying ‘Yes. / No.’ That’s why short answers are very commonly used.

To form the short answer, you use the first word from the question. (This is either an auxiliary verb or a form of ‘be’.)

Use the long form (he does) in affirmative answers (yes).

Use the short form (he doesn’t) in negative answers (no).

Question Affirmative Negative
Do we know him? Yes, we do. No, we don’t.
Can she see me? Yes, she can. No, she can’t.
Have they read Yes, they have. No, they haven’t.
the book?
Is he hungry? Yes, he is. No, he isn’t.