So the plural form of adjectives is not necessary in this kind of pattern. When would you use it?
Only rarely nowadays in the colloquial language, and I don't think that any are introduced on this course. Only a few adjectives have a plural form anyway.
mwyar duon is used as a specific term, though - 'blackberries'.
The audio for this sentence is missing the word 'newydd.'
Is it? I heard it. That's strange.