"May I have an apple? Yes ."
Translation:Ga i afal? Cei.
the multiple choice question also listed 'cewch' as an answer, is that the more formal/plural form of 'cei'?
No, ydy is not used as an answer with this pattern.
The Welsh answering system looks complex compared to English, but the underlying logic is not too hard - applying it in conversation is what needs a lot of practice. For the colloquial language, but leaving out various dialect variations, I think it can probably be summed up by working through the following sequence:
- Any emphatic question is answered with ie, or nage. If that does not apply then,
- Any simple past tense question is answered with do or naddo. If that does not apply then,
- Any question using a form of bod is answered with the related form of bod. (A very wide class of questions and answers!) If that does not apply then,
- Any question based on cael, mynd, dod, gwneud, hoffi/licio, gallu/medru (and perhaps a couple of others in especially common use) is answered with a form of the same verb. If that does not apply then,
- The rest, which will probably just be questions based on short-form future forms of verbs, can be answered with future forms of gwneud.
With any answer, the pronouns are excluded. So:
- Ga i afal - 'May I have an apple?' comes from cael and so we should answer with a form of cael meaning 'you may' or 'you may not':
- Cei - 'Yes, you may', or,
- Na chei - 'No, you may not'
(Any informed corrections to the logic welcomed!)
If you get lost in a live conversation, just go with ie/nage or do/naddo - people will understand you perfectly well. Or just avoid a direct yes/no answer altogether, which is actually very common in natural conversation anyway.