PE-47: Modals of Deduction: must be, can't be, might be
Thinking like a detective in English: how to say how sure you are about something you cannot see — and why 'mustn't be' is never the opposite of 'must be'.
PE-47: Modals of Deduction: must be, can't be, might be
The lights are off and nobody answers the door. You do not know what is happening — but you can guess, and English has exact words for how confident your guess is. "They must be out." "They can't be asleep, it's midday." "They might be in the garden." This is deduction, and it is one of the most grown-up things you can do in a foreign language.
In this lesson you will learn
- must be — I'm almost certain it IS
- can't be — I'm almost certain it ISN'T
- might / may / could be — it's possible
- Why the opposite of must be is can't be, never
mustn't be
1. The certainty scale
95must be
I'm almost sure it's true. He must be tired — he worked all night.
50might / may / could be
Perhaps. She might be at the library.
5can't be
I'm almost sure it's false. That can't be right.
0isn't
I know. He isn't at home — I've just seen him leave.
Afsona must be very clever — she won the olympiad.
Afsona juda aqlli boʻlsa kerak — u olimpiadada gʻolib boʻldi.
I have evidence, but I don't know her personally. That is a deduction.
2. The trap: must be and can't be
In PE-44 you learned that must means obligation, and its negative mustn't means prohibition. But in deduction, the negative of must be is completely different. It is can't be.
✓ Deduction
- He must be ill. (I'm sure he is)
- He can't be ill — I saw him running. (I'm sure he isn't)
✗ Not deduction
He mustn't be ill.- That would mean "he is forbidden to be ill" — which makes no sense.
3. Guessing about actions, not just states
Deduction is not limited to be. Put any base verb after the modal — and use be + -ing when you are guessing about what is happening right now.
Jasur isn't answering his phone. He must be sleeping, or he might be studying. He can't be at school — it's Sunday.
Jasur telefonini olmayapti. Uxlayotgan boʻlsa kerak yoki oʻqiyotgandir. Maktabda boʻlishi mumkin emas — bugun yakshanba.
She must know the answer — she's read the whole book.
U javobni bilsa kerak — butun kitobni oʻqib chiqqan.
With a stative verb like know, no -ing is possible (PE-13).
4. How to build a deduction
- Find your evidence. What can you actually see or know? The car isn't here.
- Decide how sure you are. Almost certain? Possible? Impossible?
- Choose the modal: must be · might/may/could be · can't be.
- Add the evidence with "because" or a dash — a deduction sounds much better with its reason: They must have gone out — the car isn't here.
5. Common mistakes
He mustn't be at home — his car has gone.
He can't be at home — his car has gone.
She must to be very rich.
She must be very rich.
That doesn't can be true.
That can't be true.
He must being tired.
He must be tired. / He must be feeling tired.
He can't be knowing the answer.
He can't know the answer. (stative verb)
Practice
1 Complete: The light is on in her room, so she ? at home.
Show answer
must be — the light is your evidence, so you are almost certain.
2 Complete: You've just eaten a whole plate of plov — you ? hungry!
Show answer
can't be — the evidence makes it almost impossible. Never
mustn't be.
3 What is the difference? (a) You must be quiet. (b) You must be tired.
Show answer
(a) is an obligation — I am telling you to be quiet. (b) is a deduction — I am guessing about how you feel. Same word, two jobs.
4 Make a deduction with evidence: Sherbek's bag is here but he isn't in the classroom.
Show answer
Model: He must be somewhere in the school — his bag is still here. He can't have gone home.
5 Order these from most to least certain: might be · can't be · must be
Show answer
must be (95%) → might be (50%) → can't be (5%).
Can't be is at the bottom because it is certainty in the negative direction.
Key words — Kalit soʻzlar
- Deductionxulosa, taxmin
- Evidencedalil
- Certain / sureishonchi komil
- Must beboʻlsa kerak
- Can't beboʻlishi mumkin emas
- To guesstaxmin qilmoq
- Impossibleimkonsiz
- Olympiadolimpiada
- Reasonsabab
- Contextkontekst, matn
🎯 Remember this
- must be (95%) → might/may/could be (50%) → can't be (5%).
- The opposite of must be is can't be — never
mustn't be. - must has two jobs: obligation and deduction. Context decides.
- Add be + -ing to guess about right now: He must be sleeping.
- Always give the evidence — a deduction needs its reason.