If someone asks you a question, you can give a long answer but you can also keep it short!
When you give a short answer, you say YES or NO + hulpwerkwoord or to be:
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, they haven't.
Is he hungry? - Yes, he is. - No, he isn't.