-(으)ㄹ걸 그랬다 — Should have, I wish I had

This expression is used when the speaker wants to express regret or a feeling of lament about something he or she should have done, or did but should not have done. When referring to something that should have been done but was not, -(으)ㄹ걸 그랬다 is used, and when referring to something that was done, but in hindsight should not have been done, either -지 말걸 그랬다 or 안 -(으)ㄹ걸 그랬다 is used.

Details

This word 그랬다 in this expression can be shortened to simply -(으)ㄹ걸. This is an Informal form of the expression used only with those with whom the speaker is close or when talking to oneself.

가: 우리 과 친구들이 여행 간다는데 너도 같이 갈래?

나: 나도 가고 싶은데 돈이 없어. 이럴 줄 알았으면 돈을 좀 아껴 쓸걸.

The expression -(으)ㄹ걸 can also be used to express a supposition. However, this suppositional -(으)ㄹ걸 is spoken with a rising intonation and is only used in informal speech. (See Chapter: '을걸요 Expressing Conjecture and Supposition' here)

가: 아키라 씨, 오셨어요? 나: 아, 모두들 정장을 입고 왔네요. 저도 정장을 입고 올걸 그랬어요.

A: Akira, have you arrived? B: (Yes,) but everyone is dressed formally. I should have worn my dress clothes.

가: 컴퓨터가 30%나 세일을 하네요, 나: 진짜요? 이렇게 세일할 줄 알았으면 조금만 기다릴걸 그랬어요. 지난주에 샀거든요.

A: Computers are a surprising 30% off. B: Really? If I had known they were having such a big sale, I would have waited a bit longer. I bought one last week.

오늘 학교 축제에 안 갔는데 유명한 연예인들이 왔다고 해요. 축제에 갈걸 그랬어요.

Today, I didn’t go to the school festival, but I heard that a lot of famous stars attended. I should have gone to the festival.