助动词（auxiliary）主要有两类：基本助动词（primary auxiliary）和情态助动词（modal auxiliary）。 

I． 要点
助动词本身无意义,在句中帮助主要动词构成一定的时态,语态、语气,或是帮助构成否定句和疑问句,
常用的助动词有 be, do, have, shall(should), will(would).

(要注意，be有可能是系动词。) 

She was crying. （cry是不及物动词，构成主谓句型，而was是助动词。不是谓语。）

She was sad. （系动词，构成主系表句型，本身是谓语的部分，即是谓语。）

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情态动词（Modal  verbs）本身有一定的词义，表示语气的单词。但是不能独立作谓语，只能和动词原形一起构成谓语。情态动词用在行为动词前，表示说话人对这一动作或状态的看法或主观设想。
情态助动词基本的有十四个：may,might; can,could; will,would; shall,should; must,need,dare,used to,ought to.had better 

1、can 能,可以,表说话人同意,许可还可表客观条件许可,如:You can go now.
提建议或请求时可用can I, can you表客气,如Can I buy you a drink?
can和be able to表能力时的区别.
can表一般具有的能力,be able to表在特定条件下的能力,如:Although the driver was badly hurt, he was able to explain what had happened.

2、may
（1）、可以,表说话人同意,许可或请求对方许可.You may go.
（2）、（现在和将来）可能,也许,只用于肯定句和否定句中,如He may not be right.

3、must, have to
must表主观上的必须,have to表客观上的必须,
如:It's getting late. I have to go. 
Must I go now. -Yes, you must.
(No, you needn't./ No, you don't have to.)

4、need, dare 这二词有实意（义）动词和情态动词两种词性,如用作实意动词后接动词不定式to do,
如用作情态动词后接动词原形.Need I go now? --Yes, you must./No, you needn't.)

How dare you say that?  你怎么敢那么说呢？ 
How dare you?  你怎么敢？
I dare you to… 我谅你不敢……

实义动词：
Will you dare the journey? 你敢旅行吗?

5、shall用于第一人称疑问句中表说话人征求对方的意见或向对方请求,如,Shall we begin our lesson?
用于二、三人称陈述句,表说话人命令、警告、允诺、威胁等口吻,如: You shall fail if you don't work harder.

6、should表应该,意为有责任,有义务.如:We should try our best to make our country more beautiful.

7、will表有做某事的意志、意愿、安心、打算,如 "Will you lend me your book?" "Yes, I will.",

8、should have done表应该做而未做
must have done表对过去事实的肯定推测
could have done表本可以做某事

9、判断句：肯定句用must, 否定句用can't, 不太肯定用may, might
He must be in the office now.
He must have gone to bed, for the light is out.
He can't be in the office. He is at home.
He couldn't have cleaned the classroom, because he didn't come here today.
He might be in the office, I am not sure.
He might have cleaned the room, I suppose.

10、 ought to 应该
Yes, I think you ought to.  是的，我想你应该去。 
With her help, he ought to have no problem.  有了她的帮助， 他应当不会有什么问题。 
There ought to be something you like, Elizabeth.  伊丽莎白，这儿应该有你喜欢的

11、had better 最好
 you had better go out into the wide world  你最好离开到外面广阔的世界去。
 I think we had better go back to work. 我想我们最好现在回到工作岗位上继续工作。
 

 系动词
 不能在句中独立作谓语，必须和后面的表语一起构成句子的谓语。（注意，是一起构成谓语。说明还是谓语。但助动词的be就不是谓语。）
 常见类型归纳：be动词（am，is，are），

感官动词（feel，look，smell，sound，taste，seem），“变得”词（get，become，turn，grow），“保持”（keep，stay）。
 
 He felt ill yesterday. 　他昨天病了。（felt是系动词，后跟表语，说明主语情况。）
 He fell off the bike.　 他从自行车上摔下来。（实义动词）
 

 He looked a perfect fool．他看上去是个十足的神经病
 Please look at the blackboard．请看黑板。