Please, can anyone explain why there is a mutation of the verb in these two sentences?
“Beth mae Katy yn ddweud?”
“Pa un mae Oliver yn ddewis?”
I didn’t think that verbs mutated after ‘yn’.
I’d be very grateful if anyone could explain this to me. I suppose there is a missing ‘ei’ or something which causes the mutation, but why would the ‘ei’ be there?
Well, technically both of those are ‘wrong’ - where did you hear/read them?
In speech, you’ll sometimes get a swallowed ‘ei’, but I wouldn’t say it’s very common in that kind of context…
Prynhawn da,
I could be wrong, dim ond dysgwr anfedrus ydwyf.
I think that this is an example of the relative form of “bod” (the relative clause) (which).
In formal Welsh, the sentences would include the particle “y” (yr before a vowel) and the pronoun “ei”.
For example:
y llyfr y mae John wedi ei ddarllen ‘the book John has read’
y llyfr yr oedd John yn ei ddarllen ‘the book John was reading’
y llyfr yr oedd John wedi ei ddarllen ‘the book John had read’
y llyfr y bydd John yn ei ddarllen ‘the book John will read’
The same structure is seen is questions at times- like the ones you have cited above.
Beth (y) mae Katy yn (ei) ddweud? (what is it which Katy is saying?)
Pa un (y) mae Oliver yn (ei) ddewis (which one is it that Oliver is choosing?)
Beth (yr) ydych chi’n (ei) wneud? (What is it which you are doing?)
Pwy (y) maen nhw’n (ei) gefnogi? (Who is it which they support?)
In more informal Welsh, the particle and pronoun is emitted, yet the mutation stays.
I should also say that the pronoun might spark different mutations depending on the gender of the predecessor:
Dyma’r ystafell y mae John yn ei pheintio (this is the room which John is painting)- an aspirate mutation is seen as ystafell is a singular feminine noun.
Dyma’r llyfrau y mae e wedi eu prynu (these are the books which he has bought)- “eu” is used as llyfrau is a plural noun.
They are from Gareth King’s “Basic Welsh: A Grammar and Workbook”. If they are wrong, how should they really read?
Ah, in that case, yes, it’s definitely about the implied ‘ei’ - um, isn’t it, @garethrking?!
They are right.
Normally of course yn would not mutate a following VN, and the reason, as @aran spots, that
Pa un mae Oliver yn ddewis?
is right is the same reason that
Beth mae Donald yn wneud?
is right, namely that there is an underlying ei which is often dropped in normal or rapid speech.
Interesting. Is this basically because the form of the question does not follow normal Welsh word order (verb first)?
Why is there supposed to be an ‘ei’ in those sentences? Those ‘ei’ seem to be everywhere, but I don’t understand why and when I am supposed to put them in a sentence.
I do not have access to the sentences - but basically the possessive adjectives ei, eu etc are used in front of the VN when you want a pronoun object of the verb. They’re not essential in speech, because the actual pronoun object (i.e. fe, hi, nhw etc) is added after the VN anyway. But more formal writing often does it with possessive adjective only.
Grammar §114 if you’ve got it, @Lena_P 
Oops @Lena_P ! You mean the sentences I just gave?
In those cases the possessive adjectives are there to refer back the previously stated object of the verb.
Again, in natural more rapid speech they are often dropped, but with the mutation remaining, like a sort of guilty little afterthought… 
Thank you so much for you kind help! I’ve got your grammar so I will read the paragraph you referred to.
And yes, I meant the sentences you gave. How would they translate - if you translated the ‘ei’? Because as far as I can understand, there would be no pronoun object of the verb in English in those sentences.
Good point. Literally, then, they translate as follows:
Beth mae Katy yn (ei) ddweud?
What is Katy saying (it)
Pa un mae Oliver yn (ei) ddewis?
Which one is Oliver choosing (it)?
Bonkers, of course, from an English point of view - because English doesn’t have a rule demanding that objects before the verb have to be restated! 
I’ve never come across another language that does it, not in my mother toungue (Swedish) and not in any other language I have studied.
Thank you so much! I think I’ve got it now, at least as far as understanding it is concerned, and in time I will probably be able to use it as well.
I wonder if you could think of the English like this:
“What is it that Katy is saying?”
and
“Which one is it that Oliver is choosing?”
A bit contrived, admittedly, and perhaps not quite the same meaning, but I wonder if it helps to make slightly more sense of the Welsh way of saying it.
Yes - it might help for some. Although I am always myself inclined to think that one should avoid trying to ‘make sense’ of what is essentially (in this case, at least) simply a different sentence structure.
What I mean is: these things don’t have to be ‘explainable’ to be understood. For the learner, it is almost easier to simply learn the grammatical rule and move on without trying to analyse.
Although I do understand that some people find enjoyment in the ‘analysing’!
Gareth,
Not a welsh question, but I wanted to ask a linguist a question. I just came across a method described as a way of romanisation of Chinese using a method called Gwoyeu Romantzyh. The great or not so great wikipedia says other systems for Chinese use diacritics, but GR spells the tones of the same vowel e.g. ai, air, ae and ay. The name is accredited to Yuen Ren Chao and I’ve looked him up and been trying to find out where he might have thought up the term Gwoyeu Romantzyh and I can’t find anything - Gwoyeu looks like a very odd word and attracted my attention. He spent time as a translator for Bertrand Russel, which I thought was also interesting and that’s my token Welsh connection.
Yes, Gwoyeu Romatzyh (no N, by the way) is the Yale system, not much used now. Yuen Ren Chao was the prof there - he also wrote a definitive grammar of the language, using his system.
It was indeed devised with the idea of showing the different tones in the spelling rather than with diacritics, and the rationale behind this was that in this way you could more clearly visually differentiate words identical except for tone, by giving each a different ‘word-shape’, and this would assist with learning. There are other similar systems. But none of them have caught on, because the pinyin system with diacritics is simpler and less bewildering to the eye, albeit with a diacritic on (almost) every word. And today pinyin rules the world.
There’s an old adage in Sinologist circles: every senior Professor of Chinese spends his first year in post devising a new system of romanisation for the language, and then the rest of his life battling for its acceptance.
I met the great Sinologist Joseph Needham several times - he was the Master of my college when I was at Cambridge. I remember sitting (slightly awestruck!) in the bar with him one evening, as he drew Chinese characters on the beermats to explain the principles of the logographic script. What an astounding man!
Thanks! That way of thinking helps me.