Modales de deducción en presente (must, can't, might)

Cuando analizas los indicios y sacas una conclusión, estás haciendo una deducción. En esta lección nos centramos en tres modales especialmente útiles para deducciones sobre el presente: must, can't y might.

She hasn't slept in two days. She must be exhausted. (casi con toda seguridad es verdad)
He says he passed the test without studying. He can't be telling the truth. (casi con toda seguridad no es verdad)
The lights are off — she might be asleep already. (posible, pero no sabemos)

No son suposiciones al azar, sino conclusiones lógicas a partir de lo que observas.

La estructura clave: sujeto + must / can't / might + infinitivo → It must be cold. She can't know. He might be home.

Los tres modales de un vistazo

Modal	Lo que expresa	Ejemplo
must	casi certeza positiva	She must know* — she's been working here for years.*
can't	casi certeza negativa	That can't be* right — I checked the figures myself.*
might	posibilidad abierta	They might be* stuck in traffic.*

Como todos los modales, estos van seguidos del infinitivo sin to — sin -s, sin -ing directamente después del modal.

Must: sacar una conclusión positiva

Se usa must cuando los indicios apuntan tan claramente en una dirección que estás casi seguro de que algo es cierto.

You've been on your feet all day. You must be tired.
She speaks six languages. She must enjoy learning languages.
He looks just like Tom. They must be related.

Es fácil confundir el must de deducción con el must de obligación («You must hand in your assignment on time»). El contexto los distingue: el must deductivo se basa en observaciones e indicios, no en reglas. Para la obligación, consulta Have to / must (obligation).

Can't: sacar una conclusión negativa

Se usa can't cuando los indicios hacen algo casi imposible — es el espejo negativo de must.

The museum closes at 5, and it's already 6. It can't be open.
This can't be the right address — there's nothing here.
He can't know the answer — it was announced after he left.

Importante: para una deducción negativa se usa can't, no mustn't. ✅ It can't be true. — ❌ It mustn't be true. En español solemos expresar esta deducción negativa con no puede ser («eso no puede ser cierto»). En inglés, la forma normal es can't; mustn't se entiende como prohibición («no debes / no se permite»).

Might: una posibilidad abierta

Se usa might (o could) cuando realmente no estás seguro — los indicios no inclinan la balanza.

She's not answering. She might be in a meeting.
It might mean something different in that context — I'm not sure.
He might know the answer — it's worth asking.

Might es más débil que must. Decir She must be in a meeting suena mucho más seguro. Con might, dejas la pregunta abierta.

Para profundizar en el uso de may, might y could para expresar posibilidad (en lugar de deducción), consulta Modals of possibility (may, might, could).

Deducciones sobre lo que está ocurriendo ahora

Cuando quieres deducir que una acción está en curso en este momento, usas must / can't / might + be + verbo -ing:

The music is so loud — they must be having a party.
He's only wearing a T-shirt. He can't be feeling very warm.
She's not at her desk. She might be having lunch.

Deducción	Forma	Ejemplo
casi seguro que está ocurriendo	must be + -ing	They must be waiting* for us.*
casi seguro que NO está ocurriendo	can't be + -ing	She can't be sleeping* — the lights are on.*
quizás está ocurriendo	might be + -ing	He might be working* late — no one can reach him.*

Escala de certeza: deducción y posibilidad

Certeza	Modal	Ejemplo
casi segura (positiva)	must	She must know* — she's the expert.*
abierta	might / could	She might know* — it's worth asking.*
casi segura (negativa)	can't	She can't know* — she just arrived.*

Must y can't expresan ambos una alta confianza — must para una conclusión positiva y can't para una negativa. Might queda en el medio, cuando los indicios no deciden.

Errores frecuentes

❌ It mustn't be true — the numbers don't add up. → ✅ It can't be true. (mustn't = prohibición; para deducción negativa: can't)
❌ She must knows the answer. → ✅ She must know the answer. (infinitivo tras modal — sin -s)
❌ He must be know the answer. → ✅ He must know the answer. (no se añade be antes de un infinitivo)
❌ They must work in there right now. → ✅ They must be working in there right now. (acción en curso: must be + -ing)
❌ She can't to be serious. → ✅ She can't be serious. (sin to tras un modal)

Comprueba lo que sabes

Elige el modal que mejor complete cada deducción.

The exam was three hours long, she finished in forty minutes, and she got every question right. She ____ be very good at maths. (must / can't / might)
I can hear voices inside. Someone ____ be home. (must / can't)
His cash is always in his wallet, and his wallet is at home. He ____ be carrying any cash. (must / can't)
She's wearing headphones, but the screen is turned away from us. She ____ be listening to music. (must / might)

Ver respuestas

She must be very good at maths. (velocidad y exactitud — indicios muy claros)
Someone must be home. (las voces son una prueba directa)
He can't be carrying any cash. (el dinero está en la cartera y la cartera no está con él)
She might be listening to music. (es posible, pero no podemos verlo confirmado)