Cath wyn should be accepted as well. An explanation of the feminine form of some adjectives is in the section notes.
See the reply above and also the course notes.
Some adjectives have feminine forms and gwyn/gwen is one of them. These days the feminine forms are not always used.
If there is no "y" at the beginning of the sentence, why does the adjective "gwen" undergo a soft mutation?
y causes weak soft mutation of a following feminine noun:
- cath, y gath - a cat, the cat
Independently, a feminine noun causes soft mutation of a following adjective. In this example, ci is masculine and cath is feminine:
- ci gwlyb, cath wlyb - a wet dog, a wet cat
Combining the two cases:
- y ci gwlyb, y gath wlyb - the wet dog, the wet cat
Here, it is the feminine noun - not the article y - that has caused mutation of the adjective.
Adjectives undergo soft mutation after a feminine singular noun, whether that noun has the article in front of it or not.
If the article is present, then the feminine singular noun itself undergoes mutation as well.
So "the white cat" would be y gath wen and "a white cat" is cath wen.