-았/었어야 했는데 — Should have, Ought to have, I regret not doing

This expression is used to indicate regret or lament about the fact that a necessary action was not completed or that some necessary state was not achieved.

Details

The expression -았/었어야 했는데 can also be used at the ends of sentences. In such cases, the original part that would have followed -았/었어야 했는데 is omitted.

가: 아이들에게 줄 크리스마스 선물 샀어요?

Did you buy any Christmas presents to give the kids?

나: 아니요, 오늘 가니까 선물이 다 팔렸더라고요. 미리 사러 갔어야 했는데…

No, when I went today, they were all sold out. I should have gone to buy them earlier...

Here, a statement like 미리 안 간 것이 후회돼요 (I regret not going more in advance) or 어떡하죠? (What should we do) has been omitted after 사러 갔어야 했는데.

While the expressions -(으)ㄹ걸 (그랬다) and -았/었어야 했는데 are used similarly, they are different in the following ways.

가: 마크 씨, 책의 내용을 요약해 보세요. 나: 잘 모르겠습니다, 교수님. 책을 미리 읽어 왔어야 했는데 죄송합니다.

A: Mark, please summarize the contents of the book. B: Professor, I don’t know it (the content). I should have read the book in advance, and I’m sorry (that I didn’t).

가: 자야 씨가 왜 그렇게 화가 났어요? 나: 제가 자야 씨한테 뚱뚱해 보인다고 했거든요. 그런 말을 하지 말았어야 했는데 후회가 돼요.

A: Why is Jaya so angry? B: I told her she looked fat. I shouldn't have said such a thing, and now I regret it.

가: 어제 발표회는 잘 끝났어요? 나: 아니요. 발표회장이 시원했어야 했는데 더워서 그런지 중간에 나가 버리는 사람들이 많더라고요.

A: Did the conference end well yesterday? B: No, the conference venue was supposed to be cooled (air conditioned), and (probably) because of the heat, a lot of people left midway through.