В пособии рассматриваются основные моменты английской грамматики: части речи имя существительное, имя прилагательное и наречие; видо-временные формы глагола; модальные глаголы; неличные формы глагола

6. Could as an Alternative to May/Might

1. Could is used for future possibility to give the idea that something is just
possible, but not particularly likely.
Compare: We may go climbing this summer. (It will possibly happen)
We could go climbing this summer, but I doubt if we’ll have time.
2. Present possibility. May, might and could are used to say that something
is possibly true at the moment of speaking.
You may/might be right.
You could be right, but I don’t think you are

7. Can in Interrogative and Negative Sentences

Can is sometimes used to talk about present possibility, but only in questions
and negative sentences.

1. Who can that be at the door?

What can she possibly want?
Can it be Susan?
Can it be true?
General questions of this kind are emotional and so their application is
rather restricted. Depending on the time reference, can in this meaning is
used in combination with different forms of the infinitive.
Present: The simple infinitive is used with stative verbs and the
continuous infinitive is found with dynamic verbs.
Can it be so late?
Can she be telling lies?
Past: Can is used with the perfect infinitive.
Can she have told that?
However, the combination of can with the perfect infinitive may also
indicate an action begun in the past and continued into the moment of
speaking.
Can she really have been at home all this time?
Can she have been waiting for us for so long?
Could is also used in this way, implying more uncertainty.
Into Russian both variants, with can and could, are translated in the same
way: Неужели это правда?, Неужели она лжет? and so on.

2. It can’t be true.

She can’t be telling lies.
Sentences of this kind mean improbability and are translated into Russian
like Не может быть (невероятно), чтобы это была правда.
Note that can’t, in this sense, is the opposite of must.
It can’t be true. She must be mistaken.
In this meaning can is also used with different types of the infinitive
depending on the time reference. Could is also used in this way making
the statement less categorical.