"건데 (것인데)" is used a lot in speech both in statements and questions. It is however just one case of the more general ~ㄴ데 form (것이다 -> 것인데 -> 건데). So you need to understand the ~ㄴ데 form. ~ㄴ데 primarily connects two parts giving a background (indirect, either causal or contrasting): - 날씨도 좋은데 공원에나 가자: The weather being so good, why don't we go to a park. - 비는 오는데 우산이 없다: It's raining and we don't have an umbrella. It is also often used as a sentence ending, to make it smoother and more nuanced. ~는데 ending is similar to the plain endings (~아/어/야, ~다), but makes it sound like there's something left unsaid. - 이거 너 주려고 만든 거야: This is what I made for you - sharp and clear cut. - 이거 너 주려고 만든 건데: same, but sounds like there's more to it, whatever it might be. - 어떻게 할 거야?: What are you going to do? (a specific question - sounds like it demands an answer) - 어떻게 할 건데?: And? What do you think you'll do? (lighter, like a passing one in an ongoing conversation) - 시험날은 다가오는데..: The test day is nearing.. (and? what am I doing?). - 이래서는 안 되는데..: I shouldn't be[act] like this.. (but I can't help it). * ~ㄹ까 is one of the common question forms, with two distinct usages: 1. an objective question(to oneself): 왜 얘가 안 올까? (Why is he not coming?), 여기가 어딜까? (What is this place?). 2. to suggest an action: 우리 이제 갈까? (Shall we go now?), 같이 식사할까? (Shall we dine together?).

It roughly translates to "Could you tell me your opinion/intention on.......?" - 이 가게에서 가장 가성비 좋은 물건이 뭔데? : What do you think gives the most bang for the buck in this shop? 가성비 = 가격대 성능 비율 = price performance ratio More importantly, depending on the context/accent, the nuance might be "nitpicking" in the questioning form: 무슨 얘기를 하려는 건데?: What *the hell* are you trying to say? 뭘 어떻게 하라는 건데?: What and how *the hell* am I supposed to do?

가: 선물 하나 줄게. 내가 만든 건데. 나: 뭔데? (뭣+이+ㄴ데:뭣인데->뭔데( ㅅ and 이 are deleted.) 어떤 건데?(것+이+ㄴ데:것인데->건데( ㅅ and 이 are deleted) 가: 여기 빵 많이 갖고 왔어. 내가 만든 건데. 하나만 먹어. 나: 뭐가 제일 맛있는 건데? 네가 골라 줘.