Is Christopher a good friend of yours? Do you want to tell me something?
Yes, he is. Yes, I do.
No, he is not. / No, he isn’t.
No, I do not. / No, I don’t.
Can I give you my bag?
Yes, you can.
No, you can not. / No, you can’t.
Does she like her new pet?
Yes, she does.
No, she does not. / No, she doesn't.
Were they having fun together?
Yes, they were.
No, they were not. / No, they weren't.