It is a 'connective ending' which associates a vague questions with fact or decision of the next clause.
For example, as you said, "I don't know exactly how to use it within a sentence." can be translated as "그것을 문장 안에서 어떻게 사용하는지 나는 정확히 모르겠다."
→ how to use it within a sentence (a vague question) + I don't know exactly (decision)
I hope these answers your question.