"Mae hi'n bum munud ar hugain wedi."
Translation:It is twenty five minutes past.
Extremely literally, "Is she in five minute on twenty after".
- pump = five
- ugain = twenty
- pump ar hugain = five-on-twenty = twenty-five
- pum munud ar hugain = five minutes on twenty = twenty-five minutes
The pump turns into pum before a noun, as here -- and the noun goes after the first part of such as compound number as pump ar hugain (and is in the singular).
And the yn is not really "in" here; it just joins up the predicate to the subject, and since the predicate is a noun phrase ("twenty-five minutes past"), the first letter gets lenited: thus pum munud turns into yn bum munud and then the yn gets contracted with the hi into hi'n.