YOU SAID:
i know what u did last summer
INTO JAPANESE
私はあなたが去年の夏に何をしたか知っています
BACK INTO ENGLISH
I know what you did last summer
INTO JAPANESE
去年の夏にあなたがやったことを私は知っている
BACK INTO ENGLISH
I know what you did last summer
You've done this before, haven't you.