It must be…, it can’t be…

  • B1 · Intermediate
  • Grammar lesson
  • 60 min
  • modals of deduction

Guesses about now with must, might, could and can't: It can't be Dad's phone. It must be a teenager's. From a phone left on a bus to a game of clues, where every guess needs a reason.

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must becan't bemight be

What's in the lesson

  1. 1Warm-up
  2. 2In this lesson
  3. 3Check the conversationListening comprehension
  4. 4What does it mean?Grammar exercise
  5. 5Find the guessesGrammar exercise
  6. 6Guessing about nowGrammar rule
  7. 7How sure?Grammar exercise
  8. 8The best guessGrammar exercise
  9. 9At the airportGrammar exercise
  10. 10Find the mistakeGrammar exercise
  11. 11Say it naturallySpeaking practice
  12. 12Where am I?Speaking practice
  13. 13About youGrammar exercise
  14. 14Wrap-up

Homework

  1. 1Homework: the box at the doorGrammar exercise
  2. 2Homework: which is correct?Grammar exercise
  3. 3Homework: build the guessesGrammar exercise
  4. 4Homework: who lives here?Discussion

Two slides from the lesson

Warm-up

A few questions about you before we start.

  1. What is the strangest thing that you have ever found on a bus, a train or a plane?
  2. On a bus or a train, do you ever watch the other passengers and imagine their lives?
  3. Do you like guessing games, or do you prefer to know the answer right away?

Grammar rule

Guessing about now

must, might, could, can't

We use them when we don't know for sure, but we have a reason for our guess.

How sure are you?UseExample
Almost sure it is truemustThe lights are on. They must be home.
It is possiblemight, could (also may)Nobody is answering. They might be asleep.
Possibly notmight not (also may not)Take a sweater. The hall might not have heating.
Almost sure it is not truecan't (also couldn't)You can't be hungry already! You just ate.

The form

  • modal + base verb, the same for every person: It must be late. He must know her. (not must to know, not must knows)
  • modal + be + -ing, for something happening right now: Shh! The baby might be sleeping.
  • maybe is one word, with a normal verb: Maybe it is hers. = It might be hers.

Watch out

  • can is not for a guess: not It can be her phone, but It might / could be her phone. (can is for what is possible in general: Winter can be very long here.)
  • must for a guess is not a rule: You must be freezing! (= I'm almost sure.) You must wear a helmet. (= a rule)
  • In American English, must not is also common for a negative guess: He never wears that sweater. He must not like it. can't is stronger: it is impossible.

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