"I must not go."
Translation:Rhaid i fi beidio â mynd.
How is this constructed? I had thought "does dim rhaid i fi mynd" but it seems the correct way of saying it is based on "paid â ...".
This expression often causes problems in Welsh - English translation.
There are two different phrases:- 'Dont have to' and 'Must not' with very different constructions in Welsh.
'I don't have to go' = Does dim rhaid i fi fynd (literally:- 'There is no necessity for me to go')
'I must not go' = (Mae) rhaid i fi beidio â mynd (literally:- 'There is a necessity for me not to go' or 'I have to not go")
I'm not understanding when to use 'mynd' over 'fynd' and vice versa...
There is a fuller explanation here:- https://www.duolingo.com/skill/cy/Gai
Basically you use the softened (mutated) form of the verb following either a preposition or some other situation. Only one word is mutated after the preposition etc.
In this exercise :- I must go = Rhaid i fi fynd (the 'i' after rhaid causes the mutation.
For the negative :- I must not go = Rhaid i fi beidio (â) mynd ( the 'peidio -> beidio' catches the mutation and thus 'mynd' doesn't change)
Hope that helps.