This is one of the big questions about learning Welsh and causes much confusion although the answer is quite simple before nouns.
Mewn = 'In' + 'indefinite article' so since there isn't an 'a' in Welsh, 'mewn' in front of a noun = 'in a'
Yn = 'In' + 'definite article' = "in the'
Mewn siop = In a shop
Yn y siop = In the shop
"Mewn" is a general form of "in" so think of it meaning "in a" and "yn" is specific so think of it as "in the". So you would use "mewn" for this sentence but use "yn" for "Mae fy mrawd i'n gweithio yn Asda" which is "My brother works in Asda".
thank you! I find it interesting how different languages can have more than one term for words we don't even consider in English. :-)
It didn't accept it from me - not sure if this discussion covers multiple uses of the sentence, because in my case it was a listening exercise as mentioned below. Even so I think it's fair: the difference between 'i'n' and 'yn' isn't so great as to override people's funny accents and normal distortion. But it may be technically impossible, of course.