"Beth sy'n bod arnoch chi?"
Translation:What's wrong with you?
I'm replying because as a learner I'm trying to get the hang of this too, and I'm hoping ibisc , shwmae, EllisVaughan, or rmcode will check and correct this as necessary.
I think 'sy'n' is used when the subject of the verb is that first word - eg beth, pwy.
Pwy sy'n siarad? Who's talking?
Beth sy'n bod? What is the matter?
When that first element and the subject of the verb are not the same, i think you use 'mae':
Pwy mae Sioned yn hoffi? Who does Sioned like? ( 'pwy' and 'Sioned' are not the same.)
Beth mae Sioned yn wneud? What is Sioned doing? (And 'gwneud' mutates because of that 'lost ei'??)
When you're asking 'what is Sioned? A teacher? A singer?' I think you use 'ydy': Beth ydy Sioned? What is Sioned? (An equivative sentence: X = Y. )
Pwy ydy Sioned? Who is Sioned? (Is she Alys's mother, Owen's business partner, ...?)
Is that right? Looking forward to feedback - diolch i chi gyd, siaradwyr Cymraeg sy'n helpu ni.