~ㄴ/은/는데 corresponds to "when", "as", "while", and general participle clauses that create an adverbial clause. It connects two clauses vaguely, implying the two actions or states are going on in parallel and loosely interrelated. ex) 1) Are you going out when it is raining so heavily? => 이렇게 비가 많이 오는데 나가려고 해? 2) Holidays are coming up and I don't have any money. (in this case the connector could be "but" instead of "and" - a case of "but" connection) => 명절은 다가오는데 나는 돈이 없다. As you said a sentence can end with ~ㄴ데 too. First, the two clauses can be broken up and swapped relatively easily, which will create a sentence ending with -ㄴ데. 1) 나가려고 해? 이렇게 비가 많이 오는데? 2) 난 돈이 없다. 명절은 다가오는데. The main clause may be omitted too. 1) When it is raining so heavily? (the speaker saw him getting ready to go out, for example) => 이렇게 비가 많이 오는데? ("are you still going out?" is implied) 2) Holidays are coming up (but)... => 명절은 다가오는데... (it leaves a nuance, "what shall I do?" for example) Such elliptical sentences are also a staple of poems and song lyrics because they are softer than plain statements ending in -이다, -이야, etc, while leaving a subtle nuance. Here's couple of examples. * "사랑한 후에" by 전인권 (in case you want to see the rest) 종소리는 맑게 퍼지고 / Bell tolls ring out clearly 저 불빛은 누굴 위한 걸까 / And I wonder who those lights are for 새벽이 내 앞에 다시 설레이는데 / When another restless dawn is before me * "그런 너" by 짙은 (Zitten) 솔직해야 한다고 이야기한 건 너인데 / It was you who said we must be honest 그렇게 하겠다고 대답한 건 나인데 / And me who answered I would try In this example, there is little reason to use ㄴ데 since it is just two independent clauses connected with "and". It could have as easily been 너야, 너였어, or 너였고, but 너인데 is used for no reason but to make it sound smoother. This example shows that -ㄴ데 can be used very loosely.