The third person plural form of a verb (maen... here) is only used with the pronoun nhw in the colloquial language.
Nouns, singular or plural, use the third person singular form of verbs:
- Maen nhw'n ofnadwy - They are terrible
- Mae'r car yn ofnadwy - The car is terrible
- Mae'r ceir yn ofnadwy - The cars are terrible
- Mae'r rheina'n ofnadwy - Those are terrible