I am beginning to understand soft mutations and feminine nouns. However, in this case, gwyn appears to have become wen (instead of the expected wyn). Can anyone explain why this is so?
All adjectives used to have masculine, feminine and plural forms.
Very few of them still remain, in the case of white the forms are:- gwyn, gwen and gwynion.
The feminine form of white is still used a little, especially in older folk songs eg 'ar lan y môr' and 'cyfri geifr' https://www.youtube.com/watch?v=B-4avghxDJY https://www.youtube.com/watch?v=LgsuJfghRuw
Cath is a feminine noun and thus its adjective mutates and so 'a red cat = cath goch; a black cat = cath ddu
Thus 'a white cat' becomes 'cath wen' (feminine form of the adjective and lose the 'g' because of mutation)
Another example would be 'a white star' = seren wen
There are very few of them so no need to worry about learning a whole new category of adjectives.
Thank you very much! That all helps a lot!