The adverb form: you drop it into a sentence to say 'this is certainly true, and I am not going to argue about it'. It works like 'certainly', 'definitely' or 'without doubt', but it is more formal and more forceful — it does not just say you are sure, it says nobody could sensibly disagree. Two normal positions. Before an adjective or noun phrase: 'She is unquestionably the best player on the team.' Or at the front of a sentence, with a comma, to colour the whole statement: 'Unquestionably, the plan cost more than they told us.' It can also sit before the main verb: 'He unquestionably knew what he was doing.' You meet it in writing, in argument, in reviews, in speeches — anywhere someone wants to sound settled. Because it is so strong, it can be a warning sign: writers often reach for it exactly when the point is arguable, hoping the confidence will carry the reader past the gap. A careful writer saves it for things that really are beyond doubt. There is one more use to notice, which learners often miss: it also appears as a full answer, on its own, meaning a firm yes. 'Was it worth the money?' 'Unquestionably.' That use is a little formal and a little dry, and it sounds educated.