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.