I want to see if I got it:
mae - positive
dydy - negative
(Not sure what interrogative is)
Also, what dialect is this?
I think this sentence isn't just one dialect mae and dydy are regular throughout Wales I believe. Ydy is the interrogative, so "Mae hi'n ferch brydferth."="She is a beautiful girl." "Dydy hi ddim yn ferch brydferth."="she isn't a beautiful girl." and "Ydy hi'n ferch brydferth?"="Is she a beautiful girl?
What about negative questions, e.g. ‘Isn’t she a beautiful girl’?
Also, is there some kind of logic to latch onto here, or is it that irregular?
There is a conjugation table in the "tips and notes" area for the "Wanting 2" skill that should answer your question if I recall correctly.
So translations are disputable I suppose but "No it has not foggy" is just not English.
That's right - recently, the underlying Duo software seems to have developed a bug whereby "it's" can always be expanded to "it has" regardless of the context of the sentence. We will just have to wait for it to be fixed - it is not something that the course teams can do.