should have done (show regret; giá mà)

* Grammar1: Verb + ㄹ/을 걸 그랬다. should have done (show regret) (Note: 걸 = 것을)
* Grammar2: Verb + 았/었/였어야 + 하는데/했는데. should have done, lý ra, lẽ ra, đáng lẽ
* Grammar3: V/A + 았/었/였으면 좋았을 텐데요. (should have done, giá như ... thì đã tốt biết mấy) (See more -ㄹ/을 텐데) Thể hiện sự nuối tiếc, hoặc là mong ước gì đó.
1. -ㄹ/을 걸 그랬다. should have done/ I regret that I didn't... (learnkorean.360korea.com)
* -ㄹ/을 걸 그랬다 is a way to express regret by saying "I should have done..." or "I regret that I didn't do..." The last part, 그랬다 means "it was so" or "I did it."; So literally, you're saying, "I did A while I could/should have done B." Here, 걸 is short for 것을.
Examples:
* 말할 걸 그랬어요. I should have told him. (I regret that I didn't tell him.)
2. V + ㄹ/을 걸 (그랬다). should have done: (bonewso.net)
This pattern is used to express regret that you didn't do a certain action in the past.
- used only with action verbs
- the modifier suffix -ㄹ/을 can be used in other places of a sentence to indicate a milder degree of obligation (equivalent to the English "to be supposed to"). For example: 할 말을 못 했어요. I couldn't say the things that I was supposed to.
- this pattern is very colloquial (thông tục).
- is often followed by 그랬다 but this can be omitted.
Examples:
* 속이 안 좋은데, 점심을 굶을 걸 그랬어요. I feel sick to my stomach; I should have skipped lunch.
* 작년에 이 차를 수리할 걸. I should have repaired this car last year (but I didn't).
* 상수하고 의논할 걸. I should have discussed it with Sang-su (but I didn't).
3. V + ㄹ/을 걸 (그랬다). should have done (koreanwikiproject.com)
* 그랬다 in this grammar is optional.
* This should not be confused for the grammar pattern A/V + (으)ㄹ걸요.
Examples:
* 공부를 더 열심히 할 걸. I should have learned harder.
* 그 영화 재미있어? 나도 같이 볼 걸 그랬어.