Must have: -았/었을 것이다

This grammar is a clever hybrid of two parts. First, you have the past tense marker -았/었-. Second, you have the future or presumptive ending -(으)ㄹ 것이다.

When you mash them together, you get a guess about something that is already finished.

It is like looking backward through a pair of "maybe" binoculars. You are projecting a possibility onto a past event.

Usually, we use this for third parties. You are guessing about your friend, the weather, or a historical event.

If you use it for yourself, you might sound like you have temporary amnesia. "I must have eaten dinner... but I don't remember!"

In daily speech, 것이다 almost always changes to 거예요 (polite) or 거야 (casual).

1. 1Forming this is quite straightforward if you already know your past tense.
2. 2Find the verb or adjective stem. For 먹다 (to eat), the stem is 먹.
3. 3Add the appropriate past tense marker: -았-, -었-, or -했-.
4. 4Add -을 것이다 (or its spoken forms 거예요/거야).
5. 5For stems ending in ㅏ or ㅗ, use -았을 것이다. Example: 가다 → 갔을 것이다.
6. 6For stems ending in other vowels, use -었을 것이다. Example: 먹다 → 먹었을 것이다.
7. 7For 하다 verbs, use 했을 것이다. Example: 공부하다 → 공부했을 것이다.
8. 8For nouns, use 이었을/였을 것이다. Example: 학생이었을 것이다.

Use this when you have a strong reason to believe something happened. Imagine you arrive at a party and all the food is gone. You would say, "Everyone must have been hungry!"

It is also great for showing empathy. If a friend tells you they worked a 12-hour shift, you can say, "You must have been tired."

Real-world scenario: You are ordering food and the restaurant is closed. You might say, "They must have run out of ingredients."

Another scenario: You are at a job interview and the interviewer asks why a previous project failed. You might say, "The deadline must have been too short."

It is perfect for discussing history or news too. "The king must have been very wise."

You are not stating a cold fact. You are adding your perspective to the story.

Don't use this for things you know for an absolute certainty. If you saw it rain with your own eyes, just use the simple past tense 왔어요.

Using -았/었을 거예요 when you know the fact makes you sound unsure or even sarcastic.

Avoid using this for your own intentional actions in most cases. If you say "I probably went to the store," people will wonder if you hit your head.

Exceptions exist, like when you genuinely forgot something or are looking at old photos. "I must have been cute when I was five!"

Also, don't use it for future guesses. That is just -(으)ㄹ 것이다 without the -았/었- part.

Keep it strictly for the "past guess" zone.

One big mistake is forgetting the -을 part. Some people try to say 먹었 것이다, which is a grammar crime in Korea!

Another mistake is confusing it with the question form -았/었을까요?. One is a statement of your guess; the other is asking for someone else's opinion.

Using the wrong past tense marker is also common. Remember, it follows the standard 아/어 conjugation rules.

Mixing up politeness levels happens too. Don't use 거야 with your boss unless you want a very awkward Monday morning.

Some learners use it to mean "I should have." That is a different grammar entirely (-았/었어야 했다).

This grammar is specifically about what *probably happened*, not what *should have happened*.

Yes, even native speakers mess up the spelling of 거예요 vs 거에요 sometimes. (Hint: it is almost always 예요).

How is this different from -았/었나 보다? Think of -았/었나 보다 as "It looks like..."

You use -았/었나 보다 when you see direct evidence right now. Like seeing wet umbrellas and thinking "It must have rained."

-았/었을 것이다 is more about your logical deduction or inner thought. It feels a bit more subjective.

Then there is -았/었겠다. This is often used as an immediate reaction to someone's story.

If a friend says "I won the lottery," you say "좋았겠다!" (That must have been great!).

-았/었을 것이다 is slightly more detached and formal. It sounds like a considered opinion rather than an emotional reaction.

Think of -았/었겠다 as a "reflex" guess and -았/었을 것이다 as a "thoughtful" guess.