"Nac ydy, dyw hi ddim yn gallu coginio."
Translation:No, she can't cook.
It would depend whether the meaning was not able to undertake the task now or not having the ability to do it ever.
'Gallu' can be the verb 'to be able' or the noun 'ability'
The English is ambiguous here and so I've added your suggestion as a possible correct answer.
If we wanted to say 'she is unable to cook' meaning she doesn't have the ability to cook (rather than she is unable to cook.....because of time,work etc)
We would say 'does dim gallu coginio gyda hi ' = she has no cooking ability.
Oh deer. It looks like I've been misunderstanding this the whole time :(
So oes/does refers to capability
While ydy/dydy/dye refers to opportunity?
So, it would be, for e.g, "does dim gallu canu gynni hi" she can't sing she has a horrible voice, but "dyw hi ddim yn gallu canu" no she can't sing tonight because she's out of town?
And presumably "medru" functions in the same way?
Yes, this is how it would be said in the south. In the north we use "Ydy" as fully interchangeable with "Yw". In the south "Yw" is for statements whereas "Ydy" is for questions, I think it's the same in Cornish. "Usi hi lowen?, "Usi, Lowen yw hi." (I think I'm correct here, but I'm from the north, so I'm not one hundred percent sure)