Hi Tiffanyrose, "-ㄴ데" is used to talk about the situation that would/could happen in the past, current, or in the future (often used to express regret). 1. When talking about the past - 어제 일찍 잤으면 오늘 아침에 지각 안했을텐데. (=If I went to bed early yesterday, I wouldn't have been late this morning). - 많이 안다쳐서 다행이야. 그 사고 때문에 크게 다칠수도 있었을텐데. (=Thanks god you didn't get serious injury. You could have got injured seriously because of the accident). 2. When talking about the current - 어제 일찍 잤으면 지금 안뛰어도 될텐데. (=If I went to bed early yesterday, I wouldn't have to be running right now). - 그때 고민 안하고 그냥 했으면 지금 후회 안할텐데. (=If I just did it without hesitation at that time, I wouldn't regret now). 3. When talking about the future - 오늘 일찍 잤으면 내일 확실히 지각 안할텐데. (-->when it is already late to go to bed) (=If I went to bed early tonight, I wouldn't be late tomorrow for sure). - 오늘 일찍 자면 내일 확실히 지각 안할텐데. (-->when there is possibility that I can go to bed early) (=If I go to bed early today, I will not be late tomorrow for sure). --> also can be translated as "오늘 일찍 자면 내일 확실히 지각 안할거야". However by ending your sentence with "ㄴ데", you are expressing the feeling of hesitation. Hope it helps.

I See Thanks a Lot 많은 감사합니다

Yeah, it has the feeling of regret or sadness. ~, I could have finished the work already. ~, I would have eaten them all.