"May I have a cup of coffee? Yes."
Translation:Ga i baned o goffi? Cewch.
Can one use cwpan for cup here and if so, how would you say the sentence?
paned/panad is derived from cwpanaid (cupful). You would not normally hear *cwpan o de/goffi.
Another word used for 'cupful' is disglaid, from dysgl (bowl, cup), which is more often heard in south-east Wales.
Similarly, for 'a glass of water' you would use gwydriad/glasiad o ddŵr rather than *gwydr/glas o ddŵr.
The -aid endings in these words are usually pronounced as /-ed/, /-ad/ or similar, and sometimes written like that in informal writing.