YOU SAID:
How could you be so confident?
INTO JAPANESE
どのようにあなたはとても自信がありますか?
BACK INTO ENGLISH
How are you so confident?
INTO JAPANESE
あなたはどのように自信がありますか?
BACK INTO ENGLISH
How confident are you?
INTO JAPANESE
あなたはどの程度自信がありますか?
BACK INTO ENGLISH
How confident are you?
That didn't even make that much sense in English.