"The black dog does not like the three white cats."
Translation:Dydy'r ci du ddim yn hoffi'r tair cath wen.
Because cath is feminine.
That causes the use of the feminine form gwen rather than the masculine gwyn, and also causes soft mutation so the gwen turns into wen.
Only a handful of adjectives still have separate feminine forms.
The use of the feminine forms of adjectives is optional nowadays, but it is quite common to use them with some adjectives such as colours. However, where they follow a feminine noun, adjectives take a soft mutation.
- cath wen - using the feminine form of gwyn
- cath wyn - using gwyn
The femine forms and the mutation of an adjective after a feminine noun are both explained in the course notes.
cath is a femine noun, so it needs the feminine form of three, tair. See the notes for the section 'Numbers'.
Thanks for the explanation re tair/tri. But why, when we haven't done feminine nouns does it appear here? I've been tearing iut my hair (eek I'm going bald) trying to suss why some adjectives take mutations and why some don't. There is not enough explanation re feminine numbers/nouns before the exercises which uses them. Thus we end up as detectives trying to suss out what is happening and why?
Would it not be better to have an exercise of the use of feminine adjectives and numbers before we have to use them?
Have you read the course notes for the section 'The'? See the note on notes - at the top of this page, click on 'Topic: Welsh", then on 'Popular' and look for the sticky discussion on 'Course hints and tips'.
The notes for 'The' should be here - https://www.duolingo.com/skill/cy/The-1/tips-and-notes.