YOU SAID:
I can not believe how idiotic you are.
INTO JAPANESE
私はあなたがどれほどばかげているか信じることができない。
BACK INTO ENGLISH
I can't believe how ridiculous you are.
INTO JAPANESE
君がどんなにばかげているか信じられない。
BACK INTO ENGLISH
I can't believe how ridiculous you are.
You've done this before, haven't you.