"After you sleep you must go to school"
Translation:Ar ôl i ti gysgu, rhaid i ti fynd i'r ysgol
Why is it "i'r ysgol"? Wouldn't that mean to the school instead of to school?
This is explained in the notes, with more examples. In Welsh we use the definite article with places that we go to habitually - school, work, bed, chapel, gym, etc. - even if we don't use it in the English equivalent.