Hello. -겠다 as in 힘들겠다, 배고프겠다, 졸리겠다, can be used either 1)to show that you are sympathizing with the person you are talking to or 2) to guess how someone is feeling. For case 1) I could say '너 지금 되게 졸리겠다(you must be really sleepy).' to someone who came back from a hard day's work. This would show that I understand that this someone must be really sleepy. For case 2), '되게 배고프시겠어요. 뭐라도 드실래요?(you must be really hungry. Would you like anything to eat?), would serve as a good example. Someone asking this question is guessing that whoever they are talking to is really hungry. -텐데 as in 힘들텐데, 배고플텐데, 졸릴텐데, could be used just like -겠다 for case 1 and 2. However, depending on the way it is used it could be used to A)express doubt -politely- on something someone is about to do, or B) to show regret for something one has or has not done. So, if I were to say '지금 되게 배고플텐데, 뭐라도 먹을래.' (You must be really hungry? Would you like anything to eat?).' It would be no different from saying '지금 되게 배고프겠다. 뭐라도 먹을래? However, as I've mentioned, you could use -텐데 in a different context as well. You could use -텐데 when someone has asked for your advice on something they are about to do, and you don't think it's a good idea(case A). If -텐데 is used in sentences like '하루 5시간 동안 운동하는 건 힘들텐데,' (are you sure you want to work out for five hours a day?) Using -텐데 in this context would sound more polite, than saying something like '왜 하루 5시간 동안이나 운동을 해(why would you work out for five hours a day)? For case B, say you got an F on a Korean exam, you could say, '어제 공부를 조금만 더 했으면 F는 안받았을텐데,' meaning, 'if I had studied a little longer yesterday I wouldn't have gotten an F.' Hope this helps :)

미묘하지만 차이가 있는데요, It must be hard. or It looks hard. = 힘들겠다 (말하는 사람의 개인적인 짐작과 그에 따른 감정이 들어감) It should be hard. = 힘들텐데 (개인의 짐작이 아니라 누가 봐도 힘들 것이 예상 가능함) 에 가깝지 않나 생각합니다. 영어를 더 잘 하시는 분 영어로 예시를 들어주시면 감사하겠습니다. :)

텐데 = would 겠다 = would, will If I had been there, I would have said something. = 내가 거기있었다면, 뭐라도 말해줬을텐데. = 내가 거기있었다면, 뭐라도 말해줬겠다. It would be hard but you have to do it. = 힘들테지만, 꼭 해야 돼. = 힘들겠지만, 꼭 해야 돼. I will do that for you. = 제가 해드리겠습니다. I would do that for you(, if possible). = 제가 해드릴텐데요(, 가능한 경우라면). = 제가 해드리겠습니다, 가능한 경우라면. (In the last example, if you want to use '겠' as would, you shouldn't omit the if clause).

Or negation? I mean something that "could have been, but isn't"?

It doesn't necessarily have a negative connotation, however some cases we use 텐데 to emphasize the speaker's regret. Like "내가 그 사람이 네 오빠인 줄 알았다면 (그 사람을) 만나지 않았을 텐데." If I knew he was your brother, I wouldn't have dated him. (saying with regret)