V+ 었어야/았어야/였어야 했는데

It is attached to the stem of an action verb to mention the following clause while expressing one’s regret or sadness about what he could not have done. You have to include such things as your reason for the preceding clause, what you have done as a replacement or your words for regret, and so on.

네 결혼식에 갔어야 했는데 갑자기 해외 출장이 잡히는 바람에 못 갔어.
I should have gone to your wedding, but I couldn’t because I was caught up on a business trip abroad.

어제 왔던 손님이 유명한 아이돌 가수래. 같이 사진이라도 찍었어야 했는데 아쉽다.
The guest who came yesterday was a famous idol singer. We should have taken a picture together. Too bad.

모임 장소가 바뀐 걸 진작에 말씀드렸어야 했는데 바빠서 공지하는 걸 깜빡 잊어버렸어요.
I should have told you about the change in the meeting place, but I forgot to announce it because I was busy.

Conversation:

아차, 라면 끓일 때 수프부터 넣었어야 했는데 또 잊어버렸어.
Oh, I should have put in the soup base first when boiling ramen but I forgot again.

넣는 순서가 중요해? 면부터 넣으나 수프부터 넣으나 다 똑같은 맛 아니야?
Does the putting-in-order matter? Whether you put the noodles first or the soup base first it all tastes the same.

수프를 먼저 넣으면 면발이 더 맛있어진다고 하더라고.
I heard that if you put the soup base in first, the noodles will taste better.

국물의 간이 면발에 잘 배서 그런가?
Is it because the soup’s seasoning permeates well into the noodle?

그건 아니고 수프 때문에 물의 끓는점이 상승해서 더 높은 온도에서 끓으니까 면발이 쫄깃해지는 거래.
It’s not like that and It’s because the boiling point of the water rises because of the soup base. As the water boils at a higher temperature, the noodles get chewy.

그럼 나처럼 약간 퍼진 면발을 좋아하는 경우에는 면부터 넣으면 되겠구나.
Then if someone likes little soggy noodles like me, he can put the noodles in first.

Related words
(간이) 배다 to permeate into
끓는점 boiling point
쫄깃하다 to be chewy
퍼지다 to go soft, to go soggy

Source: Yonsei University. Korean Language Institute