Bydden nhw is the past habitual tense of bod, also used as a conditional - 'they would (be)':
- Bydden nhw'n bwyta'r pannas tasen nhw'n flasus - 'They would eat the parsnips if they were tasty'
- Bydden nhw'n ymweld â Nain bob bore Sadwrn - 'They would visit Nain every Saturday morning' - in the sense of visiting Nain as a regular thing every Saturday.
(I have just amended the names of the tenses in my previous post)
Bydden nhw... is from the past habitual tense of bod, which can also used as a conditional tense in the colloquial language.
- Pan o'n nhw'n ifanc, bydden nhw'n ymweld â hi bob dydd Sadwrn - When they were young, they would visit here every Saturday.