는지 모르다

'Don't know why / how / when'

This admits you cannot answer a question folded into the sentence: 전원도 어떻게 켜는지 모르죠? 'you don't even know how to turn it on, do you?', 왜 우는지 몰라서 'because I didn't know why the baby was crying.' There is a second, opposite use worth knowing: after 얼마나, the same shape becomes emphatic praise or complaint — 얼마나 서로 고민을 했는지 몰라요 'you have no idea how much we agonized over it.'