YOU SAID:

i guess your just somebody that i used to know

INTO JAPANESE

私が知っていたのはあなただけの誰かだと思います

BACK INTO ENGLISH

I think it was someone you only knew

INTO JAPANESE

私はそれがあなたしか知らなかった人だと思います

BACK INTO ENGLISH

I think it is a person that only you knew

INTO JAPANESE

私はあなただけが知っていた人だと思います

BACK INTO ENGLISH

I think I was the only person you knew

INTO JAPANESE

私はあなたが知っていた唯一の人だったと思います

BACK INTO ENGLISH

I think I was the only person you knew