Modals of deduction: must, might, can't
La regla
Must, might, could, and can't express logical deductions about present situations based on observable evidence.
Modals of Present Deduction (Epistemic Present Modals) are used when a speaker makes a logical evaluation, judgment, or deduction about a current situation based on observable evidence or circumstances. Degrees of Epistemic Certainty: (1) MUST (95% Positive Certainty): Used when the speaker is virtually certain that something is true because the evidence points overwhelmingly to that conclusion: "The office lights are illuminated and his car is outside; David must be at work", "She has studied architecture for six years; she must be very knowledgeable". (2) CAN'T (95% Negative Certainty / Impossibility): Used when the speaker is virtually certain that something is impossible because the evidence completely contradicts it: "That man can't be sixty years old; he looks thirty", "She can't be at home right now; I just saw her walking into the supermarket". (3) MIGHT / MAY / COULD (30–50% Real Possibility): Used when the speaker deduces that something is possible, but lacks conclusive proof to be certain: "Why isn't the train here yet? — There might be a signal delay down the line", "Don't touch that wire; it could be dangerous", "Take an umbrella; it may rain this afternoon". Formula: Subject + must / can't / might / could + BARE BASE VERB ("He must be at home", "She can't know about it").
Recuerda
- Must expresses near certainty that something is true: the lights are on, somebody must be inside.
- Can't expresses near certainty that something is impossible: she can't be thirty, she looks eighteen.
- Might, may, and could express realistic possibility: take an umbrella, it might rain this afternoon.
Error común que evitar
Never use must not to express that something is impossible: say it can't be true, never it must not be true.
Guía visual
How to read this B1 topic
At B1, grammar links ideas: condition, result, reported meaning, experience, and narrative order.
1. Name the relationship
Ask whether the sentence gives a condition, contrast, cause, report, or result.
If it rains, we will stay home.
2. Separate time from aspect
Finished event, visible result, duration, and background action use different forms.
I had left before she arrived.
3. Test both clauses
In complex sentences, the second half often proves the first half.
I wish I knew.
Modal verbs
Modals are followed by the base verb. They do not take third-person -s.
| Meaning | Present / future | Past or perfect reference | Example |
|---|---|---|---|
| ability | can | could / was able to | She can swim. |
| obligation | must / have to | had to | We must leave. |
| advice | should | should have + participle | You should rest. |
| deduction | must / might / can't | must have / might have / can't have | He must be tired. |
Contrasts that often decide the answer
Event or background
Simple forms move events forward; continuous forms show an action in progress.
I left while she was talking.
Fact or stance
Modals, hedges, and reporting structures show how certain or responsible the speaker sounds.
This may indicate a problem.
Clause or compressed phrase
Advanced writing often compresses full clauses into participle, noun, or prepositional phrases.
Having reviewed the data, we revised the plan.
Ejemplos
- 1The office lights are on and his bicycle is parked outside; David must be inside.
- 2That luxury sports car can't belong to a high school student; it costs a fortune.
- 3Take your warm coat with you; it might turn very chilly later this evening.
- 4She speaks four languages with flawless accents; she must be an interpreter.
- 5The front door is locked and the blinds are drawn; they can't be having a party.
- 6Don't eat those wild forest mushrooms; they could be poisonous and dangerous.
- 7Why isn't the bus arriving? — There might be heavy road construction ahead.
- 8He has been jogging in the summer heat for two hours; he must be very thirsty.
- 9That answer can't be correct; two plus two definitely equals four.
- 10Sophia isn't answering her desk phone; she may be attending a client presentation.
Excepciones
Can't vs. Must Not: To state that something is logically impossible, English obligatorily uses can't, NOT must not: say "He can't be French; he doesn't speak a word of it", never "He must not be French". Must not expresses prohibition (you are forbidden to do it), not logical deduction. Could vs. Couldn't: Could expresses possibility ("It could be true"), but couldn't expresses impossibility ("It couldn't be true" = it can't be true).
Errores frecuentes
(1) Using must not for impossibility: "She must not be at home; the house is dark." → "She can't be at home; the house is dark." (Impossibility takes can't). (2) Adding to after deduction modals: "He must to be tired." → "He must be tired." (Modals are followed by bare base verbs). (3) Confusing can with could/might: Using can for specific present deduction ("It can rain today" → "It might/could rain today").
Ponte a prueba
Look at three mysterious situations around you and write deductions using must, might, and can't, then practise this topic.
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