You aren’t supposed to be here.
Yes, I am or No, I am? Also yes, I am not or no, I am not?
It isn’t raining.
Yes, it is or yes, it isn’t?
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Yes, I am or No, I am? Also yes, I am not or no, I am not?
It isn’t raining.
Yes, it is or yes, it isn’t?
If the question uses a negative verb, and you are answering briefly and contradicting the assertion, it is conventional to say "Yes," and the positive verb, as shown above.
This would mean that you are accepting that you are not supposed to be here, or that it is not raining.
Both of these are correct. It is the "but" that indicates that you are accepting the speaker's negative statement as correct.
Yes, but I had to come in here because there is a riot outside.
No, but I had to come in here because there is a riot outside.
Similarly, both of these are correct:
Yes, but the forecast is for rain, so I do need my umbrella.
No, but the forecast is for rain, so I do need my umbrella.