YOU SAID:
I can't believe you did that
INTO JAPANESE
あなたがそんなことをしたなんて信じられない
BACK INTO ENGLISH
I can't believe you did that.
INTO JAPANESE
あなたがそんなことをしたなんて信じられません。
BACK INTO ENGLISH
I can't believe you did that.
That didn't even make that much sense in English.