Why is the singular "ydy" used here even though "cyfandiroedd" is plural? Is it that the copula in identification sentences doesn't inflect for number? Or is it number agreement with "beth"?
maen and ydyn are only used with nhw in positive statements:
- Maen nhw'n fawr - 'They are large'
- Maen nhw ledled Cymru - 'They are all over Wales'
- Athrawon ydyn nhw - 'They are teachers' (an emphatic statement + nhw -> ydyn)
- Cyfandiroedd ydyn nhw i gyd - 'They are all continents' (an emphatic statement + nhw -> ydyn)
And similarly in questions:
- Beth ydyn nhw? - 'What are they?', (nhw -> ydyn) but...
- Beth ydy eu henwau? - 'What are their names?' (Plural object, but not nhw -> ydy/yw)
- Ble maen nhw? - 'Where are they?' (Plural object nhw -> maen)
- Ble mae'r athrawon 'na'n dysgu? - 'Where do those teachers teach?' (Plural object, but not nhw -> mae)