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).
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 the book? Yes, they have. No, theyhaven’t. Is he hungry? Yes, he is. No, he isn’t. Mind: If ‘you’ is the subject of the question, ‘you’ must be replaced by ‘I’ or‘we’.
Do you know him? Yes, I / we do. No, I / we don’t. If the question starts with ‘are you’, ‘are’ must sometimes be replaced by ‘am’.
Are you hungry? Yes, I am. No, I’m not. But: → Yes, we are. No, we aren’t.