Previously I attempted to translate 'who owns this car?' with 'pwy sy biau'r car yma' but it was marked as wrong. Not sure why as it appears you can use 'sy' after 'pwy' as this example illustrates.
biau is an irregular and defective verb from early forms of Welsh, and it does not follow normal rules.
From a grammatical point of view, sy is unnecessary here as its meaning is included in biau. However, in the colloquial language it is usually included.
For future and imperfect meanings, fydd and oedd respectively are used between pwy and biau, so by putting sy there in the present tense the general pattern is maintained.