Why does Duolingo only accept me and you for ich und du?
I'm not that good at English, but from my understanding, "I and you" or "me and you" are both grammatically correct. With "ich" meaning "I" in German, why does Duolingo only accept "me and you" and not "I and you"?
There's a sort-of-rule (halfway between a rule of grammar and politeness) that in such a case, you put "I" last, and say "you and I" rather than "I and you". (See https://ell.stackexchange.com/questions/9308/my-sister-and-i-versus-i-and-my-sister for more details.)
(I don't know if this is Duolingo's reasoning or not, but I can't think of anything else it could be.)
They are both correct in English, but not in the same context! It depends on the situation (or case). Also, you always put the "you" bit first.
Use "You and I" when it's a subject, and "you and me" when it's an object in the sentence. In German is the same, but they follow that rule even more closely than English does.
For example, we always say "me too", whereas in some cases it should actually be "I too", but that sounds too formal in English now. In German they have "ich auch", "mich auch" and "mir auch" which all translate into "me too".