-았/었어야 했는데

Should have, Ought to have, I regret not doing

Explanation

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.