"Nac oes, does dim rhaid i fi fynd."
Translation:No, I do not have to go.
They are different meanings, just as in English:
Does dim rhaid i ti fynd - You don't have to go (=> if you don't want to)
Mae rhaid i ti beidio â fynd! - You must not go! (=> even if you do want to go, you're not going!)
why doesn't that work the other way around, i.e. in a positive statement? Why does rhaid i ti fynd... mean you must go, as well as you need to go?
On its own, rhaid means 'necessity' or 'need/obligation'. In the expression rhaid i chi... it can be used to mean 'you must...' or you have to...' (do something).
Welsh and English words do not necessarily have a one-to-one correspondence.
Thanks, it is a rookie mistake to expect exact translations and one to one relationships! I will try not to keep making it. Plus it's much more difficult without context. Diolch!
I answered "No, I do not need to go," and it was counted wrong. Are "have to" and "need to" translated differently somehow? The meaning of the sentence doesn't change between using one over the other.