"Some thieves were caught."
Translation:Gaeth lladron eu dal.
Would it be possible to say "Gaethon lladron eu dal"? Or is this form only used with the pronoun nhw?
You answered your own question.
Plural nouns are paired with the form of the verb in the third person singular in all tenses.
Mae'r lladron yn hapus = 'The thieves are happy'
Roedd y lladron yn hapus = 'The thieves were happy'
Aeth y lladron i'r lle hapus = 'The thieves went to the happy place'
Aiff/Eith (SW/NW) y lladron i'r lle hapus = 'The thieves will go to the happy place'