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).
情态动词表一定的词义，本身并不表示动作或状态，而仅仅表达说话人的态度，它在句中须和主要动词一起构成谓语，主要的情态动词有can(could), may (might), must, have to, ought to, need, dare, shall, should, will, would.
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.
（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.)
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.
例1，They _______ to walk in the street at might.
A. didn't dare B. not dared C. not dare D. dared not
解析，该题答案为A， 此空需选一动词作谓语，因为后面是to walk, didn't dare是行为动词dare过去时态的否定形式。
例2，When he was very old, Mr. Smith _______ sit for hours without saying a word.
A. would B. should C. must D. used
解析，该题答案为A, would此处表过去的倾向性，习惯性动作，意为"总是"如: When we were children, we would go swimming every summer.