Why is ''learnt'' considered to be as a wrong answer? I am not a native English speaker but i know both forms ''learned'' and ''learnt'' are correct. Actually in British English ''learnt'' is used more than the other form.
Because Duolingo mostly uses American English :P A lot of course makers add various regional alternatives, but I guess it's pretty hard to think of all of them. Just report these cases and they'll probably be added as correct answers.