"I have gone to school Monday" makes little sense in English. It should be "I went to school Monday". Also: in other exercises "went" IS accepted for "wedi mynd".
Why is it incorrect to translate this as "I went to THE school"?
It should say 'DDYDD Llun'. The contributors have missed the adverbial mutations completely!
I'm told that leaving out the mutation in this context ("on ...day") has a long history in the south and that both versions can be correct Welsh.