YOU SAID:
I don't understand why you are so stupid
INTO JAPANESE
なんでそんなにバカなのかわからない
BACK INTO ENGLISH
i don't know why i'm so stupid
INTO JAPANESE
なぜ私はそんなに愚かなのかわからない
BACK INTO ENGLISH
i don't know why i'm so stupid
You've done this before, haven't you.