YOU SAID:

Something VERY bad must happen when u don't return the book.

INTO JAPANESE

本を返却しないと、何かとても悪いことが起こるに違いありません。

BACK INTO ENGLISH

If I don't return the book, something very bad is bound to happen.

INTO JAPANESE

もしその本を返さなかったら、何かとても悪いことが起こるに違いない。

BACK INTO ENGLISH

If I don't return the book, something very bad is bound to happen.