Sy'n?

Continuing the discussion from Explanation needed: pwy, sydd, sy'n:

I’d like to know why we have to use Pwy SY’N and not Pwy yw? I can’t see how sy’n/sydd is a form of BOD… YW is also bod, isn’t it? So why can’t we say: Pwy yw siarad? (I have learnt that the formular is Pwy + sy’n + verb/adjective, but looking for a deeper explanation as to why)

If it is a relative clause, with adjective, verbal noun, adverb or indefinite noun, sy’n/sydd is used, e.g. who is ready? pwy sy’n barod? pwy sydd yn barod? who is speaking? pwy sy’n siarad? who is here? pwy sydd yma? who is a student? pwy sy’n fyfyriwr?

But before a definite noun, yw/ydy is used e.g. Who is Siôn , pwy yw Siôn? pwy ydy Sion? who is that man? pwy yw’r dyn yna? pwy ydy’r dyn yna?

Both yw and sy(dd) are forms of the verb bod, just used in different context. You can use yw/ydy only in identification sentences / questions:
Pwy ydy hi? - Who is she?
But in the question Who is talking? you are asking for the grammatical subject that is talking, and in such cases you use sydd (or its abbreviated form):
Pwy sy’n siarad?

The same thing can happen with beth:
Beth yw hyn?What is this? compared to
Be’ sy’n digwydd?What’s happening?

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