"You must not swim."
Translation:Mae rhaid i chi beidio â nofio.
Nothing that makes sense really. You can't just put ddim after (Mae) rhaid i chi.
Your two options for negating the sentence are:
(Mae) rhaid i chi beidio (â) nofio "You mustn't swim"
Does dim rhaid i chi nofio "You don't have to swim"
Hmmm "do no swim"? I don't really understand that. How could I write "do not swim" then?
Sorry I typed it on my phone. It should say "Do not swim". And in Welsh it would be "Peidiwch â nofio".
You also need the first part = rhaid i chi = 'you must'
to complete the sentence
I would say "Y mae'n rhaid i chi beidio a nofio" is more correct. Also if someone put up a sign and wanted it to say "You must not swim" in Welsh, it would be "Peidiwch nofio"?
The usual sign that you see in the wild is the more succinct 'Dim Nofio - No Swimming'. Similarly 'Dim Parcio - No Parking'.
Use of the "y" particle is a part of the formal/literary langauge which we do not aim to teach on the course. A sign for "You must not swim" would be "Mae rhaid i chi beidio â nofio" (The â can be dropped in colloquial speech, but signs are often written in more formal language), although a more likely alternative would be "Ni chanieteir nofio" (Swimming is not allowed/forbidden- although this is very formal language). "Pediwch (â) nofio would be "Do not swim".