You often talk about things you cannot see or check directly. Your friend is late: is she stuck in traffic? Your phone won't turn on: is the battery dead? English lets you show how sure you are with one small word. 'She must be stuck in traffic' means you are almost certain. 'She might be stuck in traffic' means it is only possible. 'She can't be at home' means you believe it is impossible.

Without these modals, you can only say flat facts, and flat facts can be wrong or rude. 'He is sick' sounds like you know; 'He must be sick' shows it is your conclusion from clues. This grammar makes you sound careful, honest, and natural.

This use of modals is called deduction: you look at clues and make a logical guess. The scale of certainty has three main levels. Almost sure it is true: must. Possible, maybe fifty-fifty: may, might, could. Almost sure it is NOT true: can't. Compare: 'The lights are off. They must be asleep.' 'I hear music. Someone might be having a party.' 'That can't be Tom; Tom is in Japan.'

The form follows the normal modal rule: modal + base verb. 'She might know the answer.' 'This could take a while.' But there is a detail that catches many learners. When the guess is about a state — an adjective, a noun, or a place — you still need a verb, and that verb is be. Say 'He must be tired', 'It might be a mistake', 'They could be at the station'. You cannot jump straight from the modal to 'tired' or 'at the station'; the modal needs a verb to hold on to.

For actions happening right now, use modal + be + verb-ing: 'She isn't answering; she must be driving.' 'Don't call yet; they might be eating dinner.' Again, be is the bridge between the modal and the -ing form.

Note two surprises. First, the opposite of must here is can't, not mustn't. 'He can't be hungry; he just ate' means it is impossible. 'Mustn't' is only for rules, not for deduction. Second, in this meaning could does not talk about the past; 'It could be true' is a guess about now. Also, for deduction questions we usually don't use must; instead we ask with can or could: 'Who could that be at the door?'

So the recipe is: pick your certainty level (must / might, may, could / can't), then add a base verb, or be before adjectives, nouns, places, and -ing forms.

1. 1. Her car isn't here, so she must be out.

   A clue plus 'must be' shows you are almost certain.

2. 2. This bill can't be right; we only ordered two coffees.

   'Can't be' says the situation is impossible, based on facts.

3. 3. He might be in a meeting; his door is closed.

   'Might be' marks a real possibility, not a certainty.

4. 4. The keys could be in your coat pocket.

   'Could be' guesses about now, not about the past.

5. 5. You've been travelling all day; you must be exhausted.

   Before an adjective like 'exhausted', the modal needs 'be'.

6. 6. That man may be the new manager.

   'May be' (two words) makes a guess before a noun.

7. 7. They're not answering; they must be sleeping.

   Modal + be + verb-ing guesses about an action happening now.

8. 8. It might not be true, so check the news first.

   'Might not' shows something is possibly not the case.

9. 9. This soup can't be vegetarian; I can taste chicken.

   Taste is the clue; 'can't be' rejects the idea completely.

10. 10. The flight must be delayed; the board still says 'waiting'.

    A deduction about travel: clue first, conclusion with 'must be'.

11. 11. Who could be calling at this hour?

    Questions about possibility use 'could', not 'must'.

12. 12. She speaks perfect Spanish; she must have family in Spain.

    'Must' with a base verb ('have') for a confident guess.

13. 13. Take an umbrella; it might rain later.

    'Might' with a base verb predicts a possible future event.

14. 14. He can't be over forty; he looks so young.

    'Can't be' plus a phrase of age shows strong disbelief.

The classic mistake is dropping be: 'He must at home', 'She might tired'. A modal cannot connect directly to a place, an adjective, or a noun; it needs a verb. Wrong: 'He must at home.' Right: 'He must be at home.' Test yourself: if there is no action verb in your sentence, the verb you are missing is almost always be.

The second mistake is using mustn't for impossible things: 'He mustn't be hungry; he just ate.' In deduction, the opposite of must is can't. Wrong: 'That mustn't be true.' Right: 'That can't be true.' Save mustn't for rules and prohibition only, like 'You mustn't smoke here.'

A third mistake is confusing 'maybe' and 'may be'. 'Maybe' is one word and sits at the front of a sentence: 'Maybe she is at work.' 'May be' is a modal plus be inside the sentence: 'She may be at work.' Wrong: 'She maybe at work.' Right: 'Maybe she's at work' or 'She may be at work.'

• Certainty scale: must (sure it's true) > might / may / could (possible) > can't (sure it's false).
• Before an adjective, noun, or place, always add be: 'must be tired', 'might be at work'.
• For actions happening now, use modal + be + -ing: 'She must be driving.'
• The opposite of must in guesses is can't, never mustn't.
• 'Maybe' starts a sentence; 'may be' goes after the subject.
• Give the clue, then the guess: 'The light is on, so he must be home.'