Modals of deduction: must / can't / might

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.