June 9, 2016
iawn on its own is fine for 'fine' here. When added to another adjective it acts as an intensifier, 'very':
- Da iawn, diolch - Very good, thanks
- Mae'r car yn dda iawn - The car is very good
- Mae'r car yn ddrud iawn. - The car is very expensive.
eithaf da means 'quite good', or 'pretty good' and is used much as in English.