어야만

'Only if / one has to'

-어야만 says a condition is the only one that will do, with the 만 meaning 'only, and nothing less': 멍 아가씨가 직접 서명해야만 'only if she signs it herself.' It is heavier than plain -어야, and it often carries a note of grievance about the requirement — 꼭 그 많은 나라를 다 가져가야만 속이 후련했냐?! 'did you have to take all those countries to be satisfied?!' It appears in speeches and captions as often as in conversation.

Register: emphatic