One of the units in the Mynediad course covers this construction and has the following example:
‘Pa mor dda mae e’n gallu nofio?’ I thought this would be ‘ydy e’n gallu nofio?’ as it is a question.
I understand that Pryd, Faint, Sut and Pam are always followed by ‘mae’ eg ‘ers faint mae hi’n dysgu……’
According to BBC Catchphrase, it is ‘Pa mor dda ydy e’n…’. Which is right?
Quick straw poll here - Cai and I (fluent second language speakers) would both say ‘Pa mor dda ydy e’n’ (well, ‘ydi o’n’, but hey, details 
) - and Catrin (first language speaker) would also… 
We’ve all got faintly itchy feelings about ‘mae’ behaving a bit oddly sometimes, but in terms of common usage and being understood, you’re rock solid with ‘ydy’ here…
I’m thinking it may be ‘mae’ as in the examples …
Beth ydy hyn?
Beth mae hyn yn gwneud?
Pwy ydy o?
Pwy mae o’n ei alw?
Also, you’re correct, pryd, pam, sut, and lle are always followed by ‘mae’. Edited to add Not Faint.
I got all this out of the big red book which is in work at the moment otherwise I’d post a pic of the pages.
I just looked through as many similar examples as I could find, and the majority used “ydy” or “yw”, although a few did use “mae”. A lot of the ydy/yw examples were government or university items.
I did find one slightly different bilingual example from Aberystwyth University, which I wonder, might explain it. It uses “mae”, but it comes after “y” (that) in the Welsh version. OK, you are right - it wasn’t a question.
English: …which show once more how highly students rate the quality of the teaching…
Welsh: …y’n dangos unwaith eto pa mor uchel y mae myfyrwyr yn ystyried ansawdd yr addysgu
It should be mae, but ydy is creeping in probably through influence of English - this affects even native speakers, since they speak English as well. I’ve heard a younger native speaker say Beth ydy Ffiona’n wneud? What is Ffiona doing? Wrong - but she clearly didn’t feel so.
The basic idea of the sentence is Mae’n gallu nofio, which is not a question. In Pa mor dda mae’n gallu nofio?, we are not asking Ydy e’n gallu nofio? (because the question presupposes that he can), but asking about a skill-level - in other words the question mark is linked to the Pa mor dda, not to the Mae’n gallu nofio.
