"Ydy hi'n bum munud ar hugain wedi?"
Translation:Is it twenty five minutes past?
Could you say "It has been twenty-five minutes"? or would that be different?
I think that would be different - as i think they mean "is it 25 minutes past the hour"
Pump and chwech change to pum and chwe when they appear before another word. This actually makes it easier - it's less awkward to say pum munud or chwe munud when you're talking fast!
Also, munud is feminine, which is why you see dau > dwy, tri > tair, and pedwar > pedair. Don't worry too much about that - Duolingo will probably go over it in a later skill. http://www.omniglot.com/language/numbers/welsh.htm