Beth yw eich pwnc ffefryn?
Here's a breakdown:
* Beth - What
* yw - is
* eich - your
* pwnc - subject
* ffefryn - favorite
You can also say:
Pa bwnc sy'n well gennych chi?
This is more like saying "Which subject do you prefer?"
* Pa - Which
* pwnc - subject
* sy'n - is
* well - better
* gennych - with you (possessive)
* chi - you
Both phrases are correct and convey the same meaning.