Translation:Doing this course, I learned many things about the Romanian language.
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.
i am not a native english-speaker but i think TO ATTEND means you have to be present while TO DO, means you can do it anywhere alone.
I would say "Taking this course". I'd also have thought "Luând acest curs" would be better, but I'm not a native Romanian speaker.