Have a look at the Tips and Notes for the "Work" skill - https://www.duolingo.com/skill/cy/Work :
Many building and workplaces are also feminine and are introduced with mewn, meaning in a. The difference between yn and mewn is that yn is a very specific place, whilst mewn means any old place and in a. Yn is also used when saying in the (yn y).
Along with some examples.
FINALLY you can know how to pronounce the words..... OH NO! It's Harry Potter!