Why can't I say "Tại sao". The answer is "Tai vì sao..."?
Are you sure the answer containing "Tại sao" is rejected? I check and see that everything is already in the database. Note that primary answers (displayed here) are not the only accepted answers.
Tại vì sao is accepted, but in this case 'không is not allowed to be used at the end.
I tried "Tại sao bạn không cố gắng để học?" but it was not accepted.