These comments are from 7 years ago and it still hasn't been fixed?! 'Why have you not believed me' is not very good English, IMHO. It should be 'Why didn't you believe me?' or better, 'Why don't you believe me?', although obviously that is not in the past (present perfect) tense. Presumably, despite all the millions of users Duo keeps telling me about, nobody is bothering to check these forums and updating accordingly, as almost every lesson I come across something that doesn't sound grammatically correct in English - and that's apart from all the awful Americanisms, which are not 'proper' UK English!