"Anna, how are you? Not bad."
Translation:Anna, ciamar a tha thu? Chan eil gu dona.
I think the answer to this lies in understanding English - which is grammatically irregular here!
And the reason is that - grammatically correctly - ‘gu dona’ is an adverb.
The answer part is effectively saying ‘I’m not doing badly’.
Or ‘not badly’. Badly is an adverb because it modifies the verb ‘chan eil’.
Gu + dona = badly.
Gaelic uses gu + adjective to make adverbs as a standard approach. (So in this case I think saying ‘chan eil dona’ is just wrong.)
It’s English that is irregular and hence your question:
In English, rather than using a grammatically regular adverb (badly), we use an adjective instead (bad), ie we say ‘I’m not doing bad’ or ‘not bad’.
Also applies with good/well. In standard British or Scottish English we’ve have tended to say “I’m (doing) well’ and it’s only comparatively recently that the US English ‘I’m good’ has crept in.
No. One is simply wrong in this case.
In this case we need ‘gu dona’ because we need an adverb (badly) do modify the verb and not an adjective (bad).
This is grammatically regular; it’s English that is irregular thorough use of ‘(I’m) not (doing) bad’ / ‘I’m good’!! And that confuses us.