YOU SAID:

have i done this before, i think not

INTO JAPANESE

私は前にこれをやったことがある、私はしないと思う

BACK INTO ENGLISH

I have done this before, I think I will not do it

INTO JAPANESE

私は前にこれをしたことがある、私はそれをしないと思う

BACK INTO ENGLISH

I have done this before, I think I will not do it