Modals of Deduction: must, might, can’t
We use modal verbs to make deductions — guesses based on evidence. At B1 the key three are must (I'm sure it's true), might/could/may (it's possible), and can't (I'm sure…
We use modal verbs to make deductions — guesses based on evidence. At B1 the key three are must (I’m sure it’s true), might/could/may (it’s possible), and can’t (I’m sure it’s not true).
Present deductions
| Modal | Meaning | Example |
|---|---|---|
| must | I’m sure it IS true (90%+) | The lights are on. She must be home. |
| might / may / could | it’s possible (maybe) | He might be at work. |
| can’t | I’m sure it is NOT true | He can’t be asleep — it’s noon. |
must vs can’tFor a confident negative deduction, the opposite of must is can’t, NOT “mustn’t.” “mustn’t” means prohibition (don’t do it). “He can’t be tired” = I’m sure he isn’t.
Form
All these modals are followed by the base verb (no ‘to’): must be, might know, can’t be.
Examples from EduCareer
- Aisha answers every question — she must study hard.
- Omar isn’t replying; he might be in a lesson.
- That can’t be the right answer — check it again.
Tutor tip for Arabic speakersRemember two things: (1) no “to” after the modal — say “she must be,” not “she must to be.” (2) For a sure negative guess use can’t, never “mustn’t.” “mustn’t” is a rule, not a deduction.
Common mistakes
- She must to be tired. → She must be tired.
- He mustn’t be home (deduction). → He can’t be home.
- It might to rain. → It might rain.
Practise with the 20 questions below.
Check your understanding
Answer the questions below. You will see instantly if you are right.
1."The lights are on. She ___ be home."
sure it's true → must.
2."He's not answering. He ___ be busy."
possible → might.
3."It's noon. He ___ be asleep."
sure it's NOT true → can't.
4.For a SURE negative deduction, use:
can't = sure it isn't.
5.After 'must' we use:
must be, must know.
6."She answers everything. She ___ study hard."
strong positive deduction.
7.Which is correct?
no 'to' after must.
8."That ___ be the right answer — it's clearly wrong."
sure it's false → can't.
9.'might', 'may' and 'could' all mean:
possibility.
10."He ___ be at work; I'm not sure."
uncertain → might.
11.Which is correct?
might + base verb.
12."They've been driving for hours. They ___ be tired."
strong positive guess.
13.'mustn't' means:
prohibition, not deduction.
14."You ___ be joking! That's unbelievable."
strong reaction → must.
15."She ___ know the answer; she's an expert."
sure → must.
16."He ___ be French; he doesn't speak it at all."
sure not → can't.
17."I'm not sure where she is — she ___ be at lunch."
possible → could.
18.Choose the certain-positive modal:
must.
19."This ___ be his bag — his name is on it."
evidence → must.
20."The shop's dark. It ___ be open."
sure not → can't.
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