In starting L2 C6, I find myself getting flustered over trying to understand “beth ydy” vs. “beth mae.”
Examples of the English prompts in the lesson are “What do they want?” and “What do they want to do?” The responses are “Beth ydyn nhw eisiau” and “Beth ydyn nhw eisiau gwneud?”
Why are these “beth ydy” and not “beth mae?” I know there’s a grammar rule to do with objects and identification, but these things confuse me even in English. 
Diolch!
Possibly this might help, Joanie. Still confused myself!!!
http://ssiw.pbworks.com/w/page/44701949/Ydy(yw)%2C%20mae%20or%20sy
This is the kind of question that makes my ears bleed - and I think there’s a strong clue for you that if you have complications like this in English (but speak English fine!) you don’t need to worry too much about it…
Having said that, as well as Kim’s handy link, you might enjoy:
http://www.forumwales.com/fwforum/viewtopic.php?f=34&t=6579
http://www.forumwales.com/fwforum/viewtopic.php?f=34&t=4555
Or, of course, Gareth King’s ‘Basic Welsh: A Grammar and Workbook’:
Thanks for the links - but they seem to support “Beth mae” and not “Beth ydyn”?
S’mae Joanie,
I read the Gareth King extract, and it says that the 3-way distinction (mae/sydd/ydy) only applies to the 3rd person present. Dunno about that, but as L2C6 is not dealing with the 3rd person, perhaps that answers the question as to why is Be’ ydyn nhw and not Be’ maen nhw?
But I’m no grammar person, so maybe others can clarify.
Hwyl,
Stu
In which case your best bet is to take them as an example of colloquial speech which a grammar expert would identify is ‘incorrect’
Either way, the underlying point here is that you’ll be understood whichever you use, and you’ll naturally end up using this kind of structure in a similar way to whoever you speak to most often.
I’d have assumed that it was because it was a question:
It is cold - Mae hi’n oer.
Is it cold? - Ydy hi’n oer?
I’ve not read any of the grammar links above though, so ignore me if I’ve made matters worse!
To make it relevant to your question:
I don’t understand what they want. - Sa i’n deall beth maen nhw’n moyn. (statement)
What do they want? - Beth ydyn nhwn moyn? (question)
That’s what I’d say, anyway.
That would have been my instinct too, but I don’t think it is quite correct (though your example sentences are correct). For example we have
Lle maen nhw ?
and not
Lle ydyn nhw ?
Sadly, whenever I think I understand this I find that I’ve just deluded myself.
Join the club! But you’ll definitely hear people using both of those (even though I’m sure ‘lle maen nhw’ is ‘right’).
There is a simple answer to this particular question: ‘Beth ydyn nhw eisiau’ and ‘Beth ydyn nhw eisiau gwneud’ are both wrong. No native Welsh speaker would say either of these.
The reason they are wrong is that the nhw in these sentences is the subject of the sentence (it is ‘they’ who are doing the wanting!) and so ‘mae’ is required after ‘Beth’. Ydy/ydyn would only be right in identification sentences - ‘Beth ydyn nhw?’ What are they? - but this can’t be one of those because there is a following verb (eisiau).
So cross those out, OK? They should read ‘Beth maen nhw eisiau?’ and ‘Beth maen nhw eisiau gwneud’ (or wneud).
hm…so (very approximately): “ydyn” = “are” and “mae” = “do” (not in the “gwneud” sense of “do”, but in the “do (wanting - or other verb)” sense?
Thought I’d had an insight there, but it’s now fading fast…
Retreat and re-group …
… 
Diolch, @garethrking. I had read this explanation in one of your books, but I then began ruminating on whether “What do they want?” is technically a form of an identifying question. I was less unclear about “What do they want to do?” because it just seemed a little more complicated.
joanie -
no, it can’t be an identification sentence because there is never another verb in these…ident. sentences always have just the verb ‘to be’ (in whatever tense), with a noun or pronoun on either side, both of which refer to the same thing or person. So anything with an additional verb (i.e. verbnoun linked to the verb ‘to be’, for example) can’t be ident.
mikeellwood
No, in these ‘Beth…’ sentences, the only difference between ydyn nhw and maen nhw is that ydyn nhw would have to be an an iddent. setence, while maen nhw would have to be in a sentence where in the English version ‘they’ is the subject of the sentence -
Beth ydyn nhw? What are they? (ident)
Beth maen nhw’n ddweud? What are they saying? (subject)
The are/do distinction is not relevant in Welsh, it’s all about English context -
Beth maen nhw’n ddweud? What are they saying? OR What do they say?
Remember you’ve also got sy(dd) after Beth…? for when Beth is the subject.
Beth sy’n digwydd? What is happening?
I know this is tricky, folks - keep at it!
Thanks, Gareth!
As, Gareth has said - “I know this is tricky, folks - keep at it!” -and I’m sure you will. But, out in the gwyllt/wild people will understand what your saying by context and not blink at the mistakes learners use.
Yep! it is - Keep at it!. Better not to think too much. 
Overtime though, I catch myself saying - Be sy’n digwydd, Be ydy hwn, Beth maen nhw’n deud without thinking. It sounds more like deud than ddweud to me. I spend a lot of time in Pen-y-Groes…
deud is practically standard in the Land of Gog, I am pleased to say. And for dwedwch they say dudwch.
I’m a bit confused by this:
‘Beth ydyn nhw eisiau’ and ‘Beth ydyn nhw eisiau gwneud’ are both wrong. No native Welsh speaker would say either of these.
I’m grateful that to have access to such knowledgeable advice here and I do understand and accept your explanation on what is correct (I searched for this topic precisely because the Glossika couse follows the pattern you say is correct, which was different from what I learned in SSIW Level 2). However the quoted part worries me a little: I thought Catrin was a native speaker? I know Aran was always upfront about learning Welsh as a second language, but in the intro he says Catrin is a native speaker and I’d always taken her voice as my guarantee that I was learning something natives might say even if not technically correct.
Are you sure that no native speaker would ever say this?
Thanks in advance for your reply.
Is this in Level 2 Challenge 6? Or did you hear it somewhere else?
Catrin is a native speaker from north-west Wales.
Thank you for your answer. It was level 2 for sure. I can’t remember the exact challenge, but I remember that exact structure.
Catrin is a native speaker from north-west Wales.
Yes, I remember Aran saying that at the start of the old course.