What's the difference between what I am doing now and what I am doing in English? What should we pay attention to in both affirmative and negative answers?

It makes no difference. This is all my homework now.

If you understand it as "someone asked you what you were doing just now", I am doing my homework, and the answer to my homework is the same: I am doing my homework.

Affirmation: If the answer is no, it must be confined to the past tense. What were you doing just now? If I ask you what you are doing and answer "I didn't do my homework", I will feel a little uncomfortable. So the answer is: I didn't do my homework, I did. .....

Personal point of view