-았/었을 것
Would Have, Must Have
-았/었을 것 is used to guess about a past event. It means 'must have' or 'probably did'.
How It Works
This pattern is used to express a strong guess, assumption, or logical conclusion about an event that has already happened. It's equivalent to the English expressions 'must have + p.p.' or 'would have + p.p.'.
Formation:
It's formed by attaching the past tense infix -았/었- to a verb or adjective stem, followed by -을 것.
- Stems ending in vowels ㅏ or ㅗ: Add -았을 것.
- 가다 (to go) → 갔을 것
- 보다 (to see) → 봤을 것 - Stems ending in other vowels: Add -었을 것.
- 먹다 (to eat) → 먹었을 것
- 읽다 (to read) → 읽었을 것
- 춥다 (to be cold) → 추웠을 것 (ㅂ irregular) - Stems ending in 하다: Change to 했을 것.
- 공부하다 (to study) → 공부했을 것
- 피곤하다 (to be tired) → 피곤했을 것
In sentences, it is typically followed by a form of 이다, such as -았/었을 거예요 (standard polite), -았/었을 겁니다 (formal polite), or -았/었을 거야 (informal).
This pattern implies that the speaker has a reason or some evidence for their guess, making it a stronger assumption than simply using -ㄴ/는/ㄹ 것 같다 for the past (e.g., -았/었을 것 같다).
Examples (5)
- 어제 밤늦게까지 일해서 많이 피곤했을 거예요.
You worked until late last night, so you must have been very tired.
Word by Word: Yesterday until late night because worked, a lot must have been tired.
- 벌써 1시니까 점심을 다 먹었을 거예요.
It's already 1 o'clock, so they must have all eaten lunch.
Word by Word: Already 1 o'clock so lunch all must have eaten.
The speaker is making a logical guess based on the time.
- 그 소식을 듣고 부모님이 아주 기뻐하셨을 거예요.
Hearing that news, your parents must have been very happy.
Word by Word: That news hearing, parents very must have been happy.
The honorific marker -시- is inserted before the past tense marker: 기뻐하시 + 었을 거예요 -> 기뻐하셨을 거예요.
- 제가 도착했을 때 사무실에 아무도 없었어요. 다들 퇴근했을 거예요.
When I arrived, there was no one in the office. Everyone must have gone home.
Word by Word: I when arrived, in the office anyone wasn't there. Everyone must have left work.
- 10년 전에는 이 동네가 아주 조용했을 거예요.
10 years ago, this neighborhood must have been very quiet.
Word by Word: 10 years ago-at this neighborhood very must have been quiet.
Common Mistakes
Learn This First
To express past assumptions 'would have/must have', you need past tense and modifier forms.
Related Patterns
- -(으)ㄹ 거(예요/야)Will, Probably
-ㄹ 거예요 expresses a guess or intention about the future ('will probably...'). It contrasts with -았/었을 것, which is for guessing about the past.
- -ㄴ/는/ㄹ 것 같다Seems Like, Looks Like
-ㄴ/는/ㄹ 것 같다 also expresses a guess ('it seems like...'). However, -았/었을 것 implies a stronger, more logical deduction about a past event, while -았/었을 것 같다 is a slightly softer or more general impression.
- -겠-Will, (I) Intend to
While -겠- can mean 'will', it can also express a guess, often about the present or immediate future (e.g., 맛있겠다! - That must be delicious!). It's another way to show conjecture, but -았/었을 것 is specifically for past events.