YOU SAID:

nobody know what their purpose is

INTO JAPANESE

誰も自分の目的が何であるかを知らない

BACK INTO ENGLISH

Nobody knows what their purpose is.

INTO JAPANESE

彼らの目的が何であるかは誰にも分からない。

BACK INTO ENGLISH

No one knows what their purpose is.

INTO JAPANESE

誰も彼らの目的が何であるかを知りません。

BACK INTO ENGLISH

No one knows what their purpose is.