Posibilidad: «could» usado como alternativa a «may»/»might»

1. I wonder where Tom is. – He may/might/could be in the library, (Perhaps he is in the library). I wonder why Bill isn’t here? – He may/might/could still be waiting for a bus,(Perhaps he is still waiting for a bus). Do you think the plane will be on time? – I don’t know. It may/might/could be delayed by fog, (Perhaps it will be delayed by fog). Might/Could he be waiting for us at the station? (Do you think he is waiting …?). He may/might not be driving the car himself, (Perhaps he isn’t driving the car himself); «He couldn’t be driving the car himself,» («This is impossible. He can’t drive.»)

[could be puede usarse en lugar de may/might be, también cuando be forma parte del infinitivo continuo, y cuando be es parte de un infinitivo pasivo. En la interrogativa podemos usar tanto could como might, pero en la negativa hay una diferencia de significado entre could y may/might: «He may/might not be driving the car himself» equivale a «Perhaps he isn’t driving the car himself»; «He couldn’t be driving the car himself,» en cambio, expresa una deducción negativa, significa: «This is impossible. He can’t drive.»]

2. I wonder how Tom knew about Ann’s engagement. – He may/might/could have heard it from Jack. (Perhaps he heard it from Jack). Could/Might the bank have made a mistake? Ann might not have seen Tom yesterday (perhaps she didn’t see him); Ann couldn’t have seen Tom yesterday. (deducción negativa: quizás se encontraban en ciudades diferentes).

[could + el infinitivo de perfecto de cualquier verbo puede emplearse en lugar de may/might + infinitivo de perfecto (posibilidad). En la interrogativa, como ocurría en la combinación con «be,» podemos usar tanto «could» como «might,» pero en la negativa los significados difieren: «Ann might not have seen Tom yesterday» equivale a «Perhaps she didn’t see him»; «Ann couldn’t have seen Tom yesterday» es una deducción negativa: quizás se encontraban en ciudades diferentes.]