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Can и Could

Предложение с can в настоящем времени:

Утверждение подлежащее + can + do ...
Отрицание подлежащее + cannot / can't + do ...
Вопрос Can +подлежащее + do ...?
Мы используем can в реальной ситуации в настоящем когда говорим о:
способности что-то сделать вообще.
We can see the lake from our bedroom window.
Can you speak any foreign languages?
Victor can't walk very fast.
способности что-то сделать в определенной ситуации.
I can stay with Helen if I go to Warsaw.
Can you come and see us tomorrow evening?
Simone cannot sleep in bright light.
благоприятной возможности.
I have some free time. I can help her now.
How long can you sleep if nobody bothers you?
You can't become rich if there are poor people next to you.
Ambulance can ride on red light.
Can we cross the border here?
I can't drive my sister's car.
просьбе или запросе.
Can I have a glass of water?
Can you give me a hand?
возможности, невозможности.
It can cost more then 100 Euro.
Can learning a language be a real challenge?
It can't be 10 pm. Not dark yet.

Предложения с could в прошедшем времени:

Утверждение подлежащее + could + do ...
Отрицание подлежащее + could not / couldn't + do ...
Вопрос could / couldn't +подлежащее + do ...?
В прошедшем времени мы используем could чтобы выразить общую возможность что-то сделать или разрешение на действие.
Мы часто употребляем could со словами see, hear, smell, taste, feel, remember, understand:
I was sitting very far from scene and could not hear very well.
We could do what we wanted.
Mary could swim in cold water when she was a child.
As soon as I came into the room, I could feel a sharp smell.
В специфических ситуациях, мы употребляем was/were able to или managed to или was/were allowed to:
The fire spread very quickly, but fortunately everybody was able to escape just in time.
We didn't know where the key was, but we managed to find it in the end.
I was allowed to drive my friend's car while he was away last week.
Отрицание couldn't (could not) возможно во всех ситуациях:
My grandmother couldn't swim.
We looked for the key everywhere, but we couldn't find it.
He played well, but he couldn't beat me.

Could шиироко используется в нереальных ситуациях. И в этом случае оно имеет смысл настоящего времени. Для прошедшего времени мы используем форму Perfect.
Мы употребляем could когда говорим о возможных ситуациях в настоящем или в будущем, особенно если мы делаем предложение. Значение такое же как might или may.
"What shall we do this evening?" - "We could go to the cinema."
"When you go to Paris next month, you could stay with Julia." - "Yes, I suppose I could."
The story could be true, but I don't believe in it.
I don't know what time Liz is coming. She could get here at any time.
Can тоже допустим в этих предложениях. Однпко с could в предложении меньше уверенности, как в русском могли бы.
Мы также употребляем could для нереальных действий или событий.
I'm so hungry I coud eat a horse.
This is a wonderful place. I could stay here for ever.
Мы используем перфектную форму когда говорим о прошлом.
I was so hungry I could have eaten a horse.
My reputation at work was bad, but it could have been worse.
Что-то "could have happened" означает, что это было возможно, но не произошло:
Why did you stay at a hotel when you were in Warsaw? You could have stayed with Julia.
I didn't expect that you wanted to go to the concert. I could have got you a ticket.
Nick was lucky. He could have hurt himself when he fell, but he stood up as nothing happened.
Мы используем отрицание couldn't чтобы сказать, что что-то не было бы возможно:
I couldn't live in a big city. I'd hate it.
Everything is fine right now. Things couldn't be better.
Для прошедшего времени мы употребляем couldn't have:
We had a really good holiday. It couldn't have been better.
The trip was cancelled last week. Boris couldn't have gone anyway because he was in bed.

Более подробно о нереальных ситуациях смотрите Условные предложения.

В будущем времени мы выражаем способность что-то сделать с помощью will be able to.
I'm not sure that she will be able to drive by the time she finish the course.
I will be able to speak fluently English after three years living in the US.
Для будущей возможности или разрешения мы употребляем просто can.
You can come tomorrow, it's too late now.
I'll have some free time tomorrow. I can help her then.
We can cross the border tomorrow early in the morning.