YOU SAID:

You must do the thing you think you cannot do

INTO JAPANESE

できないと思うことをしなければならない

BACK INTO ENGLISH

I have to do something I think I can not do

INTO JAPANESE

私はできないと思う何かをしなければならない

BACK INTO ENGLISH

I must do something I think can not

INTO JAPANESE

私はできないと思う何かをしなければならない

BACK INTO ENGLISH

I must do something I think can not