YOU SAID:

but why mustnt you do it

INTO JAPANESE

しかし、なぜあなたはそれをしなければならないのですか

BACK INTO ENGLISH

But why do you have to do it

INTO JAPANESE

しかし、なぜあなたはそれをしなければならないのですか？

BACK INTO ENGLISH

But why do you have to do that?

INTO JAPANESE

なすべきとやらがそれか？

BACK INTO ENGLISH

Do you have to do that?

INTO JAPANESE

なすべきとやらがそれか？

BACK INTO ENGLISH

Do you have to do that?