Why is it "were you doing" instead of "did you do"? Does Hungarian make a distinction between preterite and imperfect? If so, where is that explained?
No, there is no grammatical distinction, both should be accepted.
I reported it.
Why is "What have you done yesterday " the wrong answer?
An expression of time such as "yesterday" is usually a signal that the present perfect tense is not appropriate.
"What did you do yesterday?" would be a correct version.
What is the problem with "What did you do yesterday" ?