YOU SAID:
I cant believe youve done this
INTO JAPANESE
私はあなたがこれをやったと信じていない
BACK INTO ENGLISH
I do not believe you did this
INTO JAPANESE
私はあなたがこれをしたとは思わない
BACK INTO ENGLISH
I do not think that you did this
INTO JAPANESE
私はあなたがこれをしたとは思わない
BACK INTO ENGLISH
I do not think that you did this
That didn't even make that much sense in English.