ydy is used with nouns, pronouns and hwn, hon, hyn as follows:

Pwy	+ ydy	+ noun (or 'r + noun)
Beth		+ pronoun
Pa + noun		+ hwn/hon/hyn/hwnnw/honno/rheina

e.e.

• Pwy ydy'r darlithydd?
• Beth ydy o?
• Beth ydy hwn?
• Pa liw ydy'r car newydd?