"He is the most peaceful."
Translation:Fe yw'r mwya heddychlon.
That means 'more peaceful' rather than 'most peaceful'. Also, there is only one l in heddychlon.
Thanks for the speedy response and the clarification. I can see my mistake in using fwy rather than mwya, but I was more interested in why 'Mae o'n' didn't work. That is, would 'Mae o'n mwya heddychlon' be just as correct as 'Fe yw'r mwya heddychlon'?
Ah, right - I see you have modified your question to clarify that. Because when we use a superlative (-est) form in Welsh, we put it into an emphatic shape of sentence, with the thing that is 'best' or whatever at the front of the sentence. In this case Fe needs to come first. It will be explained in the notes for the section, I think.
Edit - the explanation is currently in the notes for 'Comparing3'