[助动词 , can的过去式]
(1)有能力或被允许做某事:I could hear voices in the next room. She couldn't
understand what I was saying. Could I look at your photos?
(2)可能的: It could be an allergy. Where could I have put my keys?
语法结构
1 could/couldn't| + 不带to的不定式
Mozart could read music when he was only three.
Before they cut down those trees,you couldn't see the neighbours.Now you can.
表示过去的一般意义的能力
There's someone at the door.~Who could that be?
It could rain:you'd better take your umbrella.
表示现在或将来某事的可能性。
Could we leave our bags in reception?
Could you turn the music down just a little?
The director asked if she could observe my class.
请求许可，提出要求
What you could do is hire a car for the day.
Your mother could stay with us,if she likes.
表示建议和提议。
2 could | + have | + 过去分词
We lost,but we could've won,if we'd played a bit better.
I locked myself out and had to climb in the window.~ That was dangerous.You could've fallen.
表示在过去可能发生，但却没有发生的事情。
Why didn't Alice come to the meeting?~I don't know.She could've forgotten.Or maybe she was busy.
推测引起或造成过去某种情形的原因。
3 could | + not | + have | + 过去分词
Who phoned Sweden last month and talked for an hour?~ It couldn't have been
Mike because he was on holiday.
对没有引起或造成过去某种情形发生作出结论。
搭配
经常跟could和couldn't连用的副词有：easily,hardly,only,never,possibly,maybe,perhaps和well.
It could easily be stress that's causing the sleep disorder.
it was so dark I could hardly see the shore.
I couldn't possibly agree to those terms.
You could maybe get a watch dog.
This subject could well come up again.
跟could和couldn't连用的动词最常见的有:be,see,do,get和hear。
They could be Irish,judging by their accent.
I couldn't see the point of learning Latin.
Could you get some chicken livers?
固定短语
'I couldn't care less' 根本不在乎
What do you want to watch?~I couldn't care less.You choose.
强调某事对你来说一点儿都不重要。
'I couldn't agree more/less'完全同意/完全不同意
If you ask me,caviar is over-rated.~ I couldn't agree more.It's just fishy black stuff.
表示强烈的同意(或不同意)。
...couldn't be better/worse 再好不好/再坏不过
What are your neighbours like?~Really nice.Couldn't be better.
强调事情是如何地好(或坏)。
'could do with'需要，想要
I could do with a drink.What's in the fridge?
表示你需要某物