는데

sets up background, or trails off meaningfully

Its core job is to lay down the situation before the real point comes: 문진에 집중해볼까 하는데... "I was going to focus on the questionnaire, but...". Speakers constantly stop right there and let the ending do the talking, so a sentence that ends in 는데 signals surprise, a mild objection, or something left unsaid: 우리가 그런 얘기 안 했는데? "but we never said that", 더 자야 하는데... "I really should sleep more". It goes on action verbs and on 있다/없다; descriptive words take 은데 instead. It is not the same as 그런데 at the start of a sentence, though the feeling of but is related.

Register: everyday spoken; the trailing use is casual