can 和 be able to
1. can / could 表示能力。
eg:  Can you drive a car?
2 表示客观实际的可能性
eg: Anything can happen in a world like this.
3 用在疑问句中，表示“惊讶，迷惑”
eg: How can you be so careless !
4 用在一般疑问句中，表示请求，命令或建议,
通常用于一，二人称
eg: Can you tell me your name ?
5 用在否定句或疑问句中表猜测
be able to do
有各种时态的变化，强调“ 成功做成某事”。
=  succeed in doing sth.
=  manage to do sth.
Eg: I was able  to flee away.
1  could  是 can 的过去式
    could 比 can 更委婉
2 can 用在否定或疑问句中表猜测
    can’t be                        对现在情况否定猜测
    couldn’t be 更委婉
    can’t have done                对过去情况否定猜测
    couldn’t have done 更委婉
Eg: You can’t be Xiaoming
      It can’t have rained yesterday.
      Our teacher is nowhever to be found. Where can he have gone?
          cannot… too/enough:
　无论怎么…也不过分；越…越好
  eg: ① You cannot be too careful.
        ② You cannot remember enough
            English words.
Cannot but + do sth:不得不，只好
  eg: ① I cannot but admire your bravery.
        ②  I could not but choose to go.
may 和 might
may
1 表示允许或请求；通常用于第一人称
Eg: May I use your phone ?
2 没有把握的推测。
Eg: He may come ,but I am not sure.
3 may 放在句首, 表示祝愿。
    May God bless you!
might
表示推测时, 不表示时态, 只是可能性比may 小。
He might be at home.
They might be having a meeting,    but I’m not sure.
固定句型： may/ might as well do
                  不妨做某事
Eg: If this method doesn’t work, you may as well try another.
have to 和 must
1. have to表示由于客观因素不得不做
    must表示说话人的主观意志, 看法
1. My brother was very ill, so I had to
    call the doctor in the middle of the
    night.
2. He said that they must work hard.
2. have to有人称、数、时态的变
  化，而must只有一种形式。
He had to look after his sister
yesterday.
3. 否定结构中 don’t have to表示“不
  必”mustn’t表示“禁止”。
You don’t have to tell him about it.　   
  你没必要把此事告诉他。
You mustn’t tell him about it.   
  你一定不要把这件事告诉他
Must 表猜测
must be  对现在情况肯定猜测
must have done  对过去情况肯定猜测
Eg: You must be Xiaoming
      It must have rained yesterday
must表猜测，反义疑问句分两种情况：
1 原句中有过去时间状语，反义疑问句用过去时
2 原句中没有过去时间状语，反义疑问句用现在完成时
3 Eg: It must have rained yesterday, didn’t it?
4 Eg: It must have rained ,hasn’t it ?
用 must提问的一般疑问句，肯定回答用must；否定回答用needn’t 或don’t have to
Eg: Must I do the work right now ?
      Yes, you must .
      No , you needn’t /  You don’t have to
must    “非要..”
        eg: -- How old are you,
Madam?
              -- If you must know, I’m twice  my son’s age.
should 和 ought to 
should 和ought to  都为“应该”的意思，可用于各种人称。
1 Should 表示责任，应该~
Eg: You should study hard.
2 should 表示“惊讶，怀疑”
Eg: You should behave so badly
3 Should    还可表示猜测，推测,是有一定依据的推测
Eg: They set out at four and they should be home now.
      They should be ready before 12 tomorrow.
be about toshould have done      本该做~
  ought to have done
Eg: You should have studied hard.
  shouldn’t have done    本不该做~
Eg: You shouldn’t have left without saying a word.
had better + do 表示"最好"
had better (not) do sth
1. It is pretty cold. You’d better put
  on my coat.
2. She’d better not play with the dog.
would 和 used to
used to 强调“曾经~ ” 
而 would只表过去的情况，过去常常~~
People used to think that the earth was flat.
She would go out for a walk in the
morning when she was in the country.
shall /  will
1  shall  用于一，三人称的一般疑问句，表示请求建议
Eg: Shall we go and take a walk down in the garden.
      Shall he come in or wait outside?
2 shall 用于二，三人称，表示“许诺，要求，命令”
Eg: You shall have the book when I finish reading it next week.
      Everybody shall be present at the meeting in time.
      You shall be punished if you don’t get your work done in time.
will
(1) 表请求，建议，用于第二人称
    eg: Will you please go with me?
(2) 表意愿，决定，允诺
    eg: ① I will never do that again.
          ② Come whenever you will.
(3) 表“必然性，习惯性，经常性”
    eg: ① Fish will die out of water.
          ② The door won’t open.
          ③  My grandpa will talk for hours if
              you give him a chance.
（4） won’t 怎么也不~
need 和 dare
作为情态动词时，两者都主要用于疑问句、否定句和条件句。
need 作实义动词时后面的不定式必须带to，而dare作实义动词用时, 后面的to 时常可以被省略。
need, want, require, worth（形容词）后面接doing可以表示被动：
need doing = need to be done
need: (needed) 做情态动词时，仅用于
              否定句和疑问句中．
(1) You needn’t come so early.
      don’t have to = needn’t
(2) – Need I finish the work today?
      -- Yes, you must.
(3) needn’t have done：不必做而做了
      eg: You needn’t have waited for me.
      need have done  本需要做而没做
(5) That is all settled. It needn’t be

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