"I don't want to compare with Hefin."
Translation:Dw i ddim eisiau cymharu ag Hefin.
â This does not change to ag before h - the course is in beta to catch these things!
More natural would be to have something or someone with which to be comparing Hefin:
Dw i ddim eisiau cymharu Hefin â Dewi (I do not want to compare Hefin with Dewi)
Dw i dim yn moyn cymharu fy hunan â Hefin (I don't want to compare myself with Hefin)