[Korean grammar] -(으)ㄹ걸요 Expressing Conjecture and Supposition

가: 혹시 마크 씨 못 보셨어요?
Did you happen to see Mark?
나: 아마 커피숍에 있을걸요. 아까 커피숍에 간다고 했거든요.
He’s probably at the coffee shop. I heard him say that he was going there earlier.

가: 어제 양강 씨도 테니스 치러 갔을까요?
Do you think Yang Gang went to play tennis yesterday?
나: 아마 안 갔을걸요. 예전에 테니스를 못 친다고 했던 것 같아요.
I doubt he would not have gone, I think he once said that he can’t play tennis.

This expression is used when the speaker states an unconfirmed assumption or supposition about a future event or something he or she is not sure about. It is used only among close friends and appears in conversational form only, not in written form.

가: 백화점에 가려고 하는데 사람이 많을까요?
I’m planning to go to the department store, but do you think there will be a lot of people there?
나: 추석 전이니까 많을걸요.
It’s right before Chuseok, so I bet there will be a lot (of people).

가: 집 앞에 있는 마트가 몇 시에 여는지 아세요?
Do you know when the little mart in front of the house opens?
나: 아마 10시쯤 열걸요. 다른 마트들이 대부분 10시에 열거든요,
I think it opens around 10 because most of the other marts open at 10.

가: 약속 시간을 좀 늦추고 싶은데 자야 씨가 출발했을까요?
I want to change our meeting to a later time, but do you think Jaya has already left  (her home)?
나: 벌써 출발했을걸요. 자야 씨 집이 멀잖아요.
I bet she’s already left. Jaya’s house is far away.

The expression -(으)ㄹ걸 can also represent the contracted form of -(으)ㄹ걸 그랬다, in which 그했다 has been omitted. As such, it can also be used to express the speaker’s feelings of regret about a past event. For this reason, you must pay special, attention to this form when it is used informally because it can mean either supposition or regret depending on the situation. (See Chapter 25 Expressing Regret, 01 -(으)ㄹ걸 그랬다.)

1. This expression can also be used to softly disagree with the listener, that is, to express the speaker’s belief that the listener’s statement or expectation is wrong or different from his or her own.
가: 아침 8시에 출발할까요?
Shall we leave at 8 in the morning?
나: 그 시간에는 길이 많이 막힐걸요.
The traffic will probably be bad at that time.

가: 아키라 씨에게 경복궁을 구경시켜 줄까 해요.
I’m planning to take Akira sightseeing at Gyeongbokgung,
나: 아키라 씨는 경복궁에는 벌써 가 봤을걸요. 외국인들이 한국에 오면 제일 먼저 가는 곳이잖아요.
I think Akira has already gone to Gyeongbokgung. It’s one of the first places foreigners visit after coming to Korea.

2. This expression is used when only the speaker has knowledge to back up a supposition or assumption. Accordingly, it can’t be used when both the speaker and listener share the same information.
가: 주말까지 벌써 매진되었네요.
It’s already sold out for the entire weekend.
나: 그러게요. 영화가 재미있을걸요. (X)
->그러게요. 영화가 재미있나 봐요.  (O)
Yeah, that movie seems popular.
The basis for the assumption that the movie is popular—that is, the fact that the tickets are sold out for the entire weekend—is shared by both people and therefore -(으)ㄹ걸요 cannot be used.

가: 이 영화가 재미있을까요?
Do you think this movie is interesting?
나: 요즘 흥행 1 위라고 하니까 재미있을걸요.
Well, it’s number 1 at the box office these days, so I reckon it’s interesting.
Here, only 나 (the speaker)  knows that the movie is number 1 at the box office, so -(으)ㄹ걸요 can be used.

3. The intonation of sentences using this expression rises slightly at the end.
가: 강원도는 날씨가 어떤지 아세요?
Do you know what the weather’s like in Gangwondo?
나: 북쪽이라 서울보다 추울걸요.
Since it’s in the north, I bet it’s colder than Seoul,

4. In terms of strength of conviction, (으)ㄹ걸요 expresses a weaker belief than -(으)ㄹ 거예요.
(1) 가: 수영 씨가 점심을 먹었을까요?
Do you think Suyeong has had lunch?
나: 먹 었을걸요. 지금 3시잖아요.
I reckon she has. It’s already 3 o’clock.

(2) 가: 수영 씨가 점심을 먹었을까요?
Do you think Suyeong has had iunch?
나: 먹었을 거예요. 아까 식당에 밥 먹으러 간다고 하더라고요.
Yeah, I’m sure she has. A while ago, she said she was going to the cafeteria to eat.

In (1), -(으)ㄹ걸요 is used because 나 (the speaker) is basing her assumption on the fact that most people have normally eaten lunch by 3 o’clock in the afternoon. 
In (2), -(으)ㄹ 거예요 is used because 나 (the speaker) is basing her assumption on the more concrete fact that Suyeong actually said she was going to eat.