Modals of deduction: must, might, can't

When you use it

You use these modals to draw a conclusion from what you can see or know, and to show how sure you are. Must means that you are almost certain because the evidence points one way: The lights are on. Tom must be at home. May, might and could mean that something is possible but you are not sure. Can't means that you are almost certain something is not true: That can't be Anna. She is in Berlin this week. These are guesses, not facts, and the modal tells the listener how strong the guess is.

How it is formed

All of these modals are followed by the base form of the verb, without to. For a guess about a present state the verb is very often be.

For an action in progress now, use the modal with be and an -ing form: He must be sleeping.

For a guess about the past, use the modal with have and the past participle. Have never changes.

Note that could not in this use means that something was impossible, not that someone was unable to do it.

Examples

• The lights are on. Tom must be at home.
• She isn't answering her phone. She might be in a meeting.
• That can't be Anna. She is in Berlin this week.
• You have been travelling all day. You must be tired.
• I may be late tonight. There is a lot of traffic.
• The keys could be in your coat pocket.
• He can't have finished already. He only started an hour ago.
• Anna must have missed the train. She is never this late.

In everyday speech must be and can't be often stand alone as reactions: That must be expensive. It can't be true.

Common mistakes

• ✗ She mustn't be at home. The lights are off.
  ✓ She can't be at home. The lights are off.
  In Ukrainian and Russian the natural phrase is не може бути / не может быть, and learners translate it with the negative of must. But mustn't in English is a prohibition (you mustn't smoke here). The negative of a deduction with must is can't.
• ✗ He must to be tired after the trip.
  ✓ He must be tired after the trip.
  Modal verbs are followed by the bare base form. To appears with have to and need to, which are ordinary verbs, but never after must, might, may, could or can't.
• ✗ She must at home. / She must is at home.
  ✓ She must be at home.
  Ukrainian and Russian have no verb in a sentence like вона, мабуть, вдома / она, наверное, дома, so the verb is easy to drop. English needs be after the modal, and it must be the base form, not is.
• ✗ It can be Tom at the door.
  ✓ It could be Tom at the door. / It might be Tom at the door.
  Може бути / может быть translates word for word as can be, but English does not use can for a guess about one specific situation. Can describes general possibility (it can be cold here in May). For a specific guess, use could, may or might. Can't, on the other hand, is fine for a specific guess, which is an irregularity you simply have to remember.
• ✗ He must has left already.
  ✓ He must have left already.
  After any modal, have stays in its base form for every subject. The subject does not change it, and neither does the past meaning.

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