助動(dòng)詞本身無(wú)意義，在句中幫助主要?jiǎng)釉~構(gòu)成一定的時(shí)態(tài)，語(yǔ)態(tài)、語(yǔ)氣，或是幫助構(gòu)成否定句和疑問(wèn)句，常用的助動(dòng)詞有be, do, have, shall(should), will(would)。

情態(tài)動(dòng)詞表一定的詞義，本身并不表示動(dòng)作或狀態(tài)，而僅僅表達(dá)說(shuō)話人的態(tài)度，它在句中須和主要?jiǎng)釉~一起構(gòu)成謂語(yǔ)， 主要的情態(tài)動(dòng)詞有can(could), may (might), must, have to, ought to, need, dare, shall, should, will, would。

1、can

能，可以，表說(shuō)話人同意，許可還可表客觀條件許可，如:You can go now.

提建議或請(qǐng)求時(shí)可用can I, can you表客氣，如Can I buy you a drink?

2、may

（1）可以，表說(shuō)話人同意，許可或請(qǐng)求對(duì)方許可。You may go.

（2）（現(xiàn)在和將來(lái)）可能，也許，只用于肯定句和否定句中，如He may not be right.

3、must, have to

must表主觀上的必須，have to表客觀上的必須，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這二詞有實(shí)意動(dòng)詞和情態(tài)動(dòng)詞兩種詞性，如用作實(shí)意動(dòng)詞后接動(dòng)詞不定式to do，如用作情態(tài)動(dòng)詞后接動(dòng)詞原形。Need I go now? --Yes, you must./No, you needn't.)

5、shall用于第一人稱疑問(wèn)句中表說(shuō)話人征求對(duì)方的意見(jiàn)或向?qū)Ψ秸?qǐng)求，如，Shall we begin our lesson?

用于二、三人稱陳述句，表說(shuō)話人命令、警告、允諾、威脅等口吻，如: You shall fail if you don't work harder.

6、should表應(yīng)該，意為有責(zé)任，有義務(wù)。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表應(yīng)該做而未做

must have done表對(duì)過(guò)去事實(shí)的肯定推測(cè)

could have done表本可以做某事

9、判斷句：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.