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)