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Cap. 15

Modal Deduction

must be · can't be · might be · formas en pasado

Deducción modal: presente

Usamos modales para hacer deducciones — conclusiones lógicas basadas en evidencia. Tres niveles principales de certeza.

must be
casi seguro que SÍ es cierto: She must be tired — she's been working all day.
might / could be
posible — quizás 50%: He might be at home.
can't be
casi seguro que NO es cierto: That can't be right — I just checked.
Ella debe de estar cansada.
→
She must be tired.
Él podría estar en casa.
→
He might be at home.
Eso no puede ser verdad.
→
That can't be true.

La escala de certeza

100% seguro
She is tired. (hecho)
~95% seguro
She must be tired. (deducción)
~50% posible
She might/could be tired.
~95% imposible
She can't be tired. (deducción)
100% no
She isn't tired. (hecho)
¡Atención! En deducción, must significa "estoy casi seguro de que es verdad", no obligación (ese es otro uso de must).

Deducción modal: pasado

Modal + have + participio pasado

modal + have + participio pasado  →  She must have been tired.
must have + PP
She must have left already — the door's locked.
might have + PP
He might have forgotten.
can't have + PP
She can't have done it — she wasn't there.
could have + PP
He could have taken a different route.
Debe de haberse ido ya.
→
She must have left.
No puede haberlo hecho.
→
She can't have done it.

must vs can't — opuestos

must be / must have
He knows everything — he must have studied a lot.
She won — she must be very talented.
can't be / can't have
He arrived in 5 minutes — he can't have taken the motorway.
That can't be her — she's in Tokyo.

Errores frecuentes

She must be studied hard.
→
She must have studied hard.
deducción en pasado → must have + PP
He can't study — he was ill. (deducción)
→
He can't have studied — he was ill.
deducción en pasado → can't have + PP
She mustn't be home. (ella no está ahí)
→
She can't be home.
deducción negativa → can't (no mustn't)
It might to rain later.
→
It might rain later.
Los modales nunca llevan "to" después.

Resumen

must be
casi seguro que SÍ
She must be exhausted.
can't be
casi seguro que NO
That can't be right.
might/could be
posible
He might be at work.
Pasado
modal + have + PP
must have left · can't have seen
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