• It expresses a vague question.
• It expresses speculation about the possibility of a certain fact occurring or whether it will be realized.

언제 갈지 모르겠어요. I don't know when we will go.
누가 할지 몰라요. I don't know who will do it.
결과가 언제 나올지 확인했어요. I checked when the results would come out.