"Dda iawn am"

Translation: Very good at

[rmcode]

It's because one of the contributors had some bizarre ideas about sensible content, it shouldn't be mutated without context and I'll delete it.

[bilbobaglin]

Why is da mutated here?

[EllisVaughan]

I think it's because it comes in the middle of a sentence normally, so "Dwi'n dda am siarad Cymraeg". (I am good at speaking Welsh) and it mutates here because it comes after a yn/'n.