[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