"-지 않을 수 없다 =? -어야 한다" I think there is a difference between the two. * -지 않을 수 없다: one cannot help but do something because of one's weakness or shortcomings. * -어야 한다: there is a compelling reason or a forceful requirement that one must do it. Some examples: - 담배를 피우지 않을 수(가) 없다: I can't help lighting up ... (= 피우지 않고는 견딜 수가 없다: can't resist the temptation) - 담배를 피워야 한다: I must [have to] light up a cigarette (weird). - 당신을 사랑하지 않을 수 없소: I can't help falling in love with you. - 당신을 사랑해야 하오: I have to fall in love with you (weird). - 그런 이상한 동물을 본 이상 믿지 않을 수 없다. (sounds natural) - 그런 이상한 동물을 본 이상 믿어야 한다. (sounds less natural to me) There are probably cases where they are interchangeable, but I think they generally have different connotations.