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)