What is wrong with Does dim rhaid i chi ffonio yfory?
That would mean "You don't have to" (= you may if you want but there is no compulsion) rather than "You must not" (= you may not do so).
I'm doing this lesson on my phone, so I apologize in advance (to kook at the notes) . May I not use dim for not? Must it be beidio?
Yes, when it is 'not' in the sense of 'not doing something' - refraining from doing something.
"Peidio" actually means "to stop or cease." As a result, the "rhaid i ti beidio" syntax can be semi-literally translated as "You must stop" or "You must cease and desist." In other words, don't do it.
Thinking of "refraining from" or "to cease" is extremely helpful. Diolch Ibisc and Ieuan.
I missed out the 'a' and it was still marked correct.
Yes, many people miss out the â in the colloquial language. It is 'Good Welsh' to use it, though.