I left out the 'mi' before 'es i' and while in previous sentences that got accepted now it got declined. Why is that? What is the difference in meaning between writing it and leaving it out? Thanks.
No the mi is entirely optional, you should be able to leave it out whenever.
I just left the 'mi' out too and it was accepted, so maybe it is fixed.
I got it wrong because I put dydd llun - why is that wrong?
It's wrong because traditionally there would have been the preposition "ar"(on) in front of the "dydd" which causes a soft mutation. Some people leave out the "ar" these days but the mutation remains.