YOU SAID:

She was a girl I think I don't know

INTO JAPANESE

彼女は私が知らないと思う女の子だった

BACK INTO ENGLISH

She was a girl I don't think I know.

INTO JAPANESE

彼女は私が知らないと思う女の子だった。

BACK INTO ENGLISH

She was a girl I don't think I knew.

INTO JAPANESE

彼女は私が知らなかった女の子だった。

BACK INTO ENGLISH

She was a girl I didn't know.

INTO JAPANESE

彼女は私が知らない女の子だった。

BACK INTO ENGLISH

She was a girl I didn't know.