-(으)ㄹ지 모르다

Use -(으)ㄹ지도 모르다 to say that something might happen or might be true, while you are not sure.

추측·판단Guessing & judging

Han: To guess about something that may already have happened, the past tense goes on the verb (갔을지도), and 모르다 stays in the present; 갈지도 몰랐어요 does not mean 'may have gone'.

Examples

민수 씨는 벌써 집에 갔을지도 몰라요.Minsu may have already gone home.

길이 막힐지도 몰라서 일찍 출발했어요.The roads might get jammed, so I left early.

오후에 비가 올지도 모르니까 우산을 챙기세요.It might rain in the afternoon, so take an umbrella.

주말이라서 그 식당에 사람이 많을지도 몰라요.It's the weekend, so that restaurant might be crowded.

Don’t mix up

-(으)ㄹ지 모르다vs-(으)ㄹ 거예요-(으)ㄹ 거예요 is a fairly confident guess; -(으)ㄹ지도 모르다 is a weaker 'maybe': 내일 비가 올 거예요 (it will probably rain) vs 내일 비가 올지도 몰라요 (it might rain).

-(으)ㄹ지 모르다vs(의문사) + -(으)ㄹ지 모르겠다With a question word or 'A or not A', it means 'I don't know whether/when', not 'might': 그 사람이 올지 안 올지 모르겠어요 (I don't know if he'll come) vs 그 사람이 올지도 몰라요 (he might come).

-(으)ㄹ지 모르다vs-(으)ㄹ 수도 있다Very close in meaning, but -(으)ㄹ 수도 있다 states that a possibility exists, while -(으)ㄹ지도 모르다 stresses the speaker's own uncertainty: 늦을 수도 있어요 (being late is possible) vs 늦을지도 몰라요 (I might be late, I'm not sure).

How to build it

V/A+(으)ㄹ지(도) 모르다

past guess: V/A+았/었을지(도) 모르다

오다 → 올지도 몰라요; 가다 → 갔을지도 몰라요; 많다 → 많을지도 몰라요

For the meaning 'might', Koreans usually add 도: -(으)ㄹ지도 모르다. With a question word or 'A or not A' (언제 올지, 올지 안 올지), the pattern instead means 'I don't know when/whether…'.
To guess about the past, put the past tense on the verb (갔을지도), and keep 모르다 in the present (몰라요). 몰랐어요 does not make the guess past.
The sentence ends as a statement (몰라요/모르겠어요); it is not used as a command or suggestion itself, but it often gives the reason for advice: -(으)ㄹ지도 모르니까 + -(으)세요.

Classic mistake

Wrong: ~~민수 씨는 벌써 집에 갈지도 몰랐어요.~~

Right: 민수 씨는 벌써 집에 갔을지도 몰라요.

To guess about something that may already have happened, the past tense goes on the verb (갔을지도), and 모르다 stays in the present; 갈지도 몰랐어요 does not mean 'may have gone'.

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