must, can't

Using must and can't for certainty and possibility

In this lesson, we look at epistemic (logical) uses of must and can't — what we say when we are sure about a conclusion, not when we talk about rules or permission.

1. Basic ideas

• must = I am (almost) sure something is true, based on evidence.
  It must be late. Everyone looks tired.
• can't = I am (almost) sure something is not true; I think it is impossible in this situation.
  He can't be at home. His car isn’t in the driveway.

We use these to make logical guesses in the present, future, and past.

2. Form (structure)

• must + base verb (no to): must be, must know, must work
• can't + base verb: can't be, can't know, can't work
• For the past, use must have + past participle and can't have + past participle:
  must have gone, can't have seen

3. Must for strong certainty (it is probably true)

3.1 Present or very near future

Use must + base verb to say you are very sure something is true now.

• She must be at work. (I’m almost sure she is at work now.)
• You must be tired. (The trip was long, so I’m sure you feel tired.)
• That must be the new manager. (I see someone new, and I conclude it’s the manager.)

3.2 With continuous and perfect forms

• must be + -ing (ongoing action now):
  They must be studying. (I hear silence; I’m sure they are studying now.)
• must have + past participle (past action):
  He must have left early. (He isn’t here now; I’m sure he left early.)

4. Can't for strong certainty something is impossible

4.1 Present or very near future

Use can't + base verb to say you are sure something is not true now.

• She can't be at work. (Her car is here; I’m sure she is not at work.)
• He can’t be serious. (I’m sure he’s joking; it’s impossible he is serious.)
• They can't know the answer. (I’m sure they don’t know.)

4.2 With continuous and perfect forms

• can't be + -ing (ongoing action now):
  They can’t be sleeping. (The lights are on; I’m sure they aren’t sleeping.)
• can't have + past participle (past action):
  She can’t have seen the email. (She didn’t reply; I’m sure she didn’t see it.)
• He can’t have done it alone. (It’s too difficult; I’m sure he didn’t do it by himself.)

5. Comparing must, can't, and possibility (may/might/could)

must and can't express strong certainty. To talk about weaker possibility, we use may, might, could.

• Strong positive certainty (≈ 95–100% sure):
  She must be at home. (I’m almost sure.)
• Strong negative certainty (≈ 0–5% sure):
  She can't be at home. (I’m almost sure she isn’t.)
• Medium possibility (maybe yes, maybe no):
  She may be at home. / She might be at home. / She could be at home.

Notice we do not use mustn't for negative certainty about facts. For logical deduction, we say can't, not mustn't.

• ✗ She mustn't be at home. (sounds like a rule: she is not allowed to be at home)
• ✓ She can't be at home. (I’m sure she isn’t there.)

6. Talking about the past

Use must have + past participle and can't have + past participle.

6.1 Must have + past participle (strong certainty about a past event)

• They must have forgotten the meeting.
  (They didn’t come; I’m sure they forgot.)
• She must have studied a lot.
  (She got an A; I’m sure she studied hard.)

6.2 Can't have + past participle (strong certainty something did not happen)

• He can't have left already.
  (His coat is still here; I’m sure he didn’t leave.)
• They can't have finished the project.
  (It’s too early; I’m sure it’s not finished.)

7. Common mistakes to avoid

• Don’t use mustn't for logical impossibility.
  Use can't instead when you mean “it’s impossible that…”
• Don’t add to after must or can't.
  ✗ He must to be tired.
  ✓ He must be tired.
• Remember the have in past forms.
  ✗ She must seen it.
  ✓ She must have seen it.

8. Quick review

• must + base verb → strong belief something is true now
• can't + base verb → strong belief something is not true now
• must have + past participle → strong belief something was true in the past
• can't have + past participle → strong belief something was not true in the past