Translation:Ym mis Mai
That yn (when it is a preposition meaning "in") causes nasal mutation:
- p becomes mh
- b becomes m
- t becomes nh
- d becomes n
- c becomes ngh
- g becomes ng
and if the result starts with
- m, yn becomes ym
- ng, yn becomes yng
- (n, yn becomes/stays yn)
So you will have, for example, ym Mhwllheli for "in Pwllheli" and ym mis ... for "in the month of ..."; the ym change happens regardless of whether the m- at the beginning of the next word was there before or whether it is a result of nasal mutation.
Minizinamo is correct, "mh" is as you would expect an "m" followed by an "h", same with "nh". As for "ngh" it's a bit different. It is indeed a combination if "ng" and "h" though the "ng" in welsh (unlike the English) doesn't re-pronounce the "g" e.g compare english Finger to German Finger and the welsh pronunciation of "ng" is the same as the german. Also it should be "ym mhwlleli" in mizinamo's post not "ym hwlleli".
I'm not sure here but I believe that traditionally, those letter combinations represented voiceless sounds (so like "m", "n", and "ng" but without vibration of the vocal cords), but that today, a pronunciation like "m+h, n+h, ng+h" is also found, e.g. effectively "ym Hwllheli".
No, mewn is unspecific and is usually translated as "in a". Compare:
Dw i'n gweithio mewn siop - "I work in a shop".
Dw i'n gweithio yn y siop - "I work in the shop".
The first sentence is an unspecific shop, the second is specific - "the shop" - which implies the listener knows which shop it must be.