면 사라지다

if X, it disappears

-(으)면 gives the condition and 사라지다 is vanishing. It covers a thing fading out of existence (35년이 되면 사라지니까, 저거 2주면 사라질 것 같아) and a person slipping away (학교 끝나면 사라져요, 'he vanishes once school ends'). One line is the word 아니면 'or else', where the -면 is not a condition at all.