Usamos modais para expressar graus de certeza sobre o presente e o passado: must (certeza positiva), can't / couldn't (impossibilidade) e might / may / could (possibilidade).
That must be John — he always wears a red hat. (tenho a certeza)
She can't be at home — her car isn't there. (é impossível)
He might be busy — try calling later. (talvez)
Para deduzir sobre o presente: modal + infinitivo.
He must be tired — he's been working all day. (certeza — lógica)
She can't be a doctor — she's only 19! (impossível — lógica)
They might know the answer — ask them. (possível)
It could be a mistake. (possível — menos certo que 'might')
Para deduzir sobre o passado, use modal + have + past participle.
She must have forgotten — she's never late otherwise. (certamente esqueceu)
He can't have heard us — the music was too loud. (impossível que tenha ouvido)
They might have left already — the door is open. (talvez já tenham saído)
As armadilhas clássicas para falantes de português
Estes são os erros mais frequentes dos falantes de português neste ponto.