"Wales is in Europe."
Translation:Mae Cymru yn Ewrop.
Mewn is a non-specific “in”, and yn (at least, in this context) is an exact “in”, so you want just one of them — not both. For instance, “Dw i'n gweithio mewn theatr” is “I work in a theatre” whereas “Dw i'n gweithio yn y theatr” is “I work in the theatr.” There is only one Europe, so you don't use mewn.
I get very confused when to use "ydy" for an affirmative statement and when to use "mae ... yn".
For example, for "Cardiff is the capital city of Wales", I put "Mae Caerdydd yn brifddinas Cymru" but this was wrong and should be "Caerdydd ydy prifddinas Cymru".
So I thought, "Okay, then here it must be "Cymru ydy yn Ewrop", but no, it's the opposite!
Can anyone explain? Thanks.
Very loosely speaking, it is because lots of places are in Europe, but only Cardiff is the capital of Wales, and Cardiff is the capital of Wales and of no other country.
A similar distinction is used when talking about someone's profession - mostly someone will have only one job, so somebody talking about me would state something like Ymgynghorydd cyfrifiadurol ydy Ian