[高考英语语法]情态动词详解
1、情态动词 + have done sth这个结构有着特殊意义：用来表示猜测（设想可能发生过什么事情）或
想象（设想可能出现过什么不同的情况）
①should (ought to) have done sth 本来该做而未做
This wall oughtn’t to have been painted blue.
②need have done sth 本来有必要做而未做
You needn’t have told her the news.
③must have done sth 对过去所做动作的肯定推测
---We went to Paris.
---That must have been nice.
④can have done sth 对过去所做动作的否定或疑问推测
I don’t think he can have heard you. Call again.
Where can John have put the matches?
⑤may (might) have done sth 过去可能发生某事，与could相比，may和 might可能性较小，might 可能性更小；might 也可表示过去可能发生而未发生的事情。
Polly’s very late----she may (might) have missed her etrain.
You were stupid to try climbing there. You might have killed youself
⑥could have done sth
●推测过去”可能“发生某事 （同can, 但can不用肯定句）
She could have gone off with some friends.
●表示过去没有实现的可能性：某事可能发生，但却没有发生
You were stupid to go skiing there----you could have broken your leg.
●表示有能力做而未做（虚拟语气）
You could have helped me !
(You were able to help me, but you didn’t.)
2、can could be able to 表示”能力“
●can 通常表示现在的或”一般的能力“---即你无论什么时候想做就能
做到的能力, 指单纯的表示一个人有某种能力。
You can certainly cook, even if you can’t do anything else.
●be able to表示某人通过努力、克服困难做成某事。
will be able to 表示将来的能力
I’ll be able to speak German in another few months.
●could 表达 ”一般的能力“---即你过去想干什么就可以干什么
She could sing like an angel when she was a kid.
但could 不能表达过去某种具体的能力，此时用was able to, managed to 或 succeeded in 等
How many eggs were you able to get ?
3、can, could, may和might 表示”可能性“
●可用来表示：①理论上的可能性（不涉及是否真发生）；②提出建议
（提出解决某个问题的可能办法或者采取的行动）；③在问句或否定句中表达现在的可能性）
Anybody who wants to can become a prison visitor.
①---What shall we do ?
---We can try asking Lucy for help. ②
Who can that be at the door ? Can it be Polly? ③
●may 可用来表达：①将来的可能性；②说话时某事可能是真实的
We may go climbing this summer. ①
You may be right. ②
●could / might 可用来表示： ①说话时某事可能是真实的；②将来
某事有可能发生，但不表示特别可能发生。
You could be right, but I don’t think you are. ①
It could rain later on this evening. ②
4、can, could, may 和might表示“允许”
①请求允许：它们都可以表示请求允许做某事,could 和 might并不是表示过去
Can (May, Could) I borrow your umbrella ?
②允许：当我们允许某人做某事时，我们用can和may来表达，而不能使用could和might
You may / can watch TV for as long as you like. (不用could, might; mustn’t 有“拒绝”的意思）
---Could I use your phone?
---Yes, of course you can /may. (No, you can’t / may not.)
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