定义
情态（情感和状态），说话人表达的情况和态度
情态动词（modal verbs) 本身有一定的词义，一般不能独立作谓语，只能和行为动词一起构成谓语，情态动词用在动词前，表示说话人对着一个动作或状态的看法或主观设想。情态动词数量数量不多，但是用途广泛。
can(could), may(might), must, need, ought to, dare(dared), shall(should), will(would), must not.
三大分类
1. 只做情态动词
must, can, may....
2. 可做情态动词有可做实义动词
need, dare
3. 可做情态动词有可做助动词
shall(should), will(would)
位置
1. 具有助动词作用，可以用来构成否定句，疑问句已经简单回答
Can you sing an English song?
Yes, I can.
2. 后接动词原形，即不带to的不定式
She may lose her way.
3. 无人称和数的变化
We must stay here.
He must stay here.
4. 有一定的词义，但并不完整，必须于动词原形一起构成谓语
He can help you.
5. could 引导的表示委婉预期的疑问句常用来请求别人帮助或对长辈的请求
Could you come to my party on Saturday afternoon?
情态动词表推测三种句式
1. 在肯定句中一般用must, can, could, might, may
He must/can/may/might know the answer.
It is cold in the room. They must have turned off the heating.
2. 否定句中用 can't /couldn't, may not / might not
It can't/couldn't be the leadmaster. He has gone to America.
He may not/might not know the scientist.
3. 疑问句中用can/could
Could he have finished the task?
Can he be at home now?
情态动词表推测三种时态
1. 对将来情况的推测，用 情态动词 + 动词原形
She must/may/might/could arrive before 5.
She must/may/might/could walk miles and miles among the hills without meeting anyone.
2. 对现在或一般情况的推测，用 ‘情态动词 + be’, ‘情态动词 + be doing’, 或 ‘情态动词 + 动词原形’
He must/may/might/could be listening to the radio.
He can't(couldn't)/may(might) not be at home at that time.
Mr.Jack is on time for everything.
How can(could) he be late for the opening ceremony.
3. 对过去情况的推测，用‘情态动词+have+过去分词’
The ground is wet. It must/may/might/could have rained last night.
The door was locked. He can(could) not/ may(might) not have been at home.
注意
情态动词should/ought to 表推测时，意为 ‘想必会，理应...’. 但与 'have + 过去分词' 连用时，则又可构成虚拟语气，意为 '本该做某事却没做'
It's seven o'clock. Jack should / ought to be here at any moment.（推测）
She should / ought to have attended your birthday party, but she had to look after her mother in hospital(虚拟）
例子
Just be patient. You can't expect the world to change so soon.
-Good morning. I've got an appointment with Miss Smith in the personnel Department.
-Ah, good morning. You must be Mrs. Peters.
- The weather has been very hot and dry.
-Yes, If it had rained even a drop, things would be much better now! And my vegetables wouldn't have died.
Mark needn't have hurried. After driving at top speed, he arrived half an hour early.
- John went to the hospital alone.
- If he had told me about it, I would have gone with him.
- Sorry, Professor Smith. I didn't finish the assignment yesterday.
-Oh, you should have done it as yesterday is the deadline.
You mustn't park here! It's an emergency exit.
'You must have a wrong number,' she said. 'There's no one of that name here.'
- I haven't got the reference book yet, but I'll have a test on the subject next month.
- Don;t worry. You shall have it by Friday.
I have told you the truth. Must I keep repeating it?