"Yes, you must go."
Translation:Oes, mae rhaid i chi fynd.
I'm not sure, I also thought "Oes, Rhaid i ti fynd" would be ok but I see now that it could be confuesed with "Oes rhaid i ti fynd?". Maybe use Mae in this case to distinguish between this and the question form.
It's a question of clarity.
The question 'do you have to go?' = oes, rhaid i ti/chi fynd?
The normal statement 'you have to go' would be 'mae/mae'n rhaid i ti/chi fynd.'
(this is literally:- 'it is necessary/there is a necessity for you to go). In normal speech the statement is contracted and drops the 'mae/mae'n'*
(*The standard is 'mae' but some dialects use 'mae'n)
When you have the answer to the question:- 'Yes, you have to go' if you don't put 'mae' in then the form is exactly as the question.
Oes, mae/mae'n rhaid i chi fynd = yes, you have to go.
OK, it could be argued that 'Oes' followed by a comma is enough of a difference.
Thus 'Oes rhaid i chi fynd?
Oes,<pause> rhaid i chi fynd!
So I will add it as an alternative.