eng8-5.2· Unit 5: Modals & Review· ~13 min

Modals of deduction: must / can't / might

Certainty and possibility in the present.

When we make a logical conclusion about the present based on evidence or clues, we use modal verbs of deduction instead of saying 'I think' or 'I know'. We use 'must' when we are almost certain something is true — the evidence strongly points to it. We use 'can't' (or 'cannot') when we are almost certain something is NOT true — it is logically impossible given what we know. We use 'might', 'may', or 'could' when we think something is possible but we are not sure either way. The structure is always modal + base verb (the infinitive without 'to'), for example: 'She must be tired' (never 'must is tired'), 'He can't be at home' (the lights are off), 'They might know the answer'. The strength of certainty goes: must (near certain true) > might/may/could (50/50 possible) > can't (near certain false). Notice that 'must' here is deduction, NOT obligation — context tells you which meaning is intended. For example: The classroom is empty and the bell has rung — the students must be in the hall; they can't be in the classroom.

Key terms

deduction — A logical conclusion you reach about a present situation based on evidence or clues, rather than direct knowledge. Modal verbs of deduction (must, can't, might) show how certain you are.
certainty — How sure you are that something is true or false. High certainty that something IS true uses 'must'; high certainty that something is NOT true uses 'can't'.
possibility — When something may or may not be true and you are not sure either way. It is expressed with 'might', 'may', or 'could'.
must (sure positive) — Used when the evidence makes you almost certain something IS true, for example: 'She must be tired — she worked twelve hours.' Here 'must' is deduction, not obligation.
can't (sure negative) — Used when the evidence makes you almost certain something is NOT true, because it is logically impossible, for example: 'He can't be at home — the lights are off.'
might / may / could (possible) — Used when something is possible but uncertain, with no strong evidence either way, for example: 'They might be in the library.' All three express the same level of possibility.
Modals of deduction: what each one signals
ModalCertaintyExample
mustSure it IS true (strong evidence)She must be the teacher — she has the register.
can'tSure it is NOT true (logically impossible)He can't be hungry — he just ate a full meal.
might / may / couldPossible, but not sure either wayThey might be in the library.
The scale of certainty
LevelModal
Certain yes (near-certain true)must
Possible (50/50, not sure)might / may / could
Certain no (near-certain false)can't
✎Picking the modal from strong positive evidence
  1. 1Read the clue: 'Kamran scored 100% on every maths test this year. He ___ be very good at maths.'
  2. 2Judge the certainty: Perfect scores all year are strong evidence that points clearly to one conclusion: he IS very good at maths.
  3. 3Match to the scale: Certain yes (near-certain true) means you choose 'must', not 'might' (too weak) and not 'can't' (the opposite).
  4. 4Build the sentence: Use modal + base verb: 'He must be very good at maths.' (not 'must is', not 'must to be').
✎Picking the modal when something is impossible
  1. 1Read the clue: 'He is only five years old. He ___ be the school director.'
  2. 2Judge the certainty: A five-year-old being a school director is logically impossible, so you are nearly certain it is NOT true.
  3. 3Match to the scale: Certain no (near-certain false) means you choose 'can't', not 'must' (the opposite) and not 'might' (too weak).
  4. 4Build the sentence: Use modal + base verb: 'He can't be the school director.'
🚫Common mistake

For a near-certain NEGATIVE deduction, use 'can't', NOT 'mustn't'. 'Mustn't' means prohibition (you are not allowed to), so 'He mustn't be at home' is wrong — say 'He can't be at home.'

⚠️Caution

Here 'must' is deduction (a conclusion from evidence), NOT obligation. 'You must be working day and night' deduces a fact; 'You must finish your homework' is a requirement. Context tells you which meaning is intended.

🚫Common mistake

Always use modal + BASE verb (the infinitive without 'to'). Say 'must be', 'might know', 'can't be' — never 'must to be', 'must is', 'might knows', or 'can't being'.

💡Note

Remember the scale: must (near-certain true) is stronger than might / may / could (just possible), which is stronger than can't (near-certain false). Match the modal to how strong your evidence is.

Rules

  1. 1Use 'must + base verb' when the evidence makes you almost certain something is true: 'She must be the teacher — she has the register.'
  2. 2Use 'can't + base verb' when the evidence makes you almost certain something is NOT true: 'He can't be hungry — he just ate a full meal.'
  3. 3Use 'might / may / could + base verb' when something is possible but not certain: 'They might be in the library.'
  4. 4The structure is always modal + BASE verb (infinitive without 'to'): say 'must be', NOT 'must is' / 'must to be'.
  5. 5The scale of certainty: must (very likely true) → might/may/could (possible) → can't (very likely false).

Practice

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