3-25. Could - сослагательность

 
1. Если контекст в предложении не указывает на то, что речь идет о прошлом, could выражает сослагательность:

It can be expensive to keep a cat (это может быть так, и так бывает).
It could be expensive to keep a cat (если бы у нас была кошка, это могло бы быть затратным).

I could never get all these clothes into that suitcase.
Is there anything I can do to help? - No, thank you, there's nothing you could do.

Разницу можно видеть и на примерах с сложноподчиненными предложениями (в прошлом и будущем):

As he was not interrupted, he finished (was able to finish, managed to finish, succeeded in finishing) the work by evening.
If people did not interrupt him, he could (would be able to) finish the work by evening.

If he is not interrupted, he can (will be able to) finish the work by evening.
If he had not been interrupted, he could have finished (would have been able to finish) the work by evening.

Как и в русском языке, сослагательность придает высказываниям более вежливый оттенок и/или неуверенность:

Could you please give me a stamp?
Where shall we go tomorrow? We could go to the restaraunt.
Come when you like. I could see you at any time.
You could go now if you like(d).
Could I borrow your pen?
Could you lend me five pounds until tomorrow?
Do you think you could help me for a few minutes?
When you've finished the washing-up you can clean the kitchen. Then you could iron the clothes, if you like.
If you haven't got anything to do you could sort out your photos.
Excuse me. Could you tell where the theatre is?
Do you think I could use your pen?
I wonder if they could help us.
Could I have the sugar, please? (за столом)
I could mend your bicycle for you, if that would help.

2. Could может выражать неудовольствие:

You could ask before you borrow my car (ты мог бы меня спросить…).