YOU SAID:
I love him but I must not
INTO JAPANESE
私は彼を愛していますが、そうしてはなりません
BACK INTO ENGLISH
I love him but he shouldn't
INTO JAPANESE
私は彼を愛していますが、彼はそうすべきではありません
BACK INTO ENGLISH
I love him but he shouldn't
You've done this before, haven't you.