ㄴ지 알다

'Know what / where / why…'

This wraps a question word inside a bigger sentence about knowing: 저희가 온 곳이 어딘지 아십니까? 'do you know where we've come to?' In conversation the short 뭔지 알아? works like the English 'you know what?' to set up what you are about to say — 근데 나는 제일 걱정되는 게 뭔지 알아 'you know what worries me most?' The question word can be anything: what, where, how much.