（十）情態(tài)動(dòng)詞與助動(dòng)詞
I．要點(diǎn)
助動(dòng)詞本身無意義，在句中幫助主要?jiǎng)釉~構(gòu)成一定的時(shí)態(tài)，語態(tài)、語氣，或是幫助構(gòu)成否定句和疑問句，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)說話人的態(tài)度，它在句中須和主要?jiǎng)釉~一起構(gòu)成謂語，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.
提建議或請求時(shí)可用can I, can you表客氣，如Can I buy you a drink?
can和be able to表能力時(shí)的區(qū)別。
can表一般具有的能力，be able to表在特定條件下的能力，如:Although the driver was badly hurt, he was able to explain what had happened.
2、may
（1）、可以，表說話人同意，許可或請求對方許可。You may go.
（2）、（現(xiàn)在和將來）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用于第一人稱疑問句中表說話人征求對方的意見或向?qū)Ψ秸埱螅纾琒hall we begin our lesson?
用于二、三人稱陳述句，表說話人命令、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表對過去事實(shí)的肯定推測
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.
II．例題
例1，They _______ to walk in the street at night.
A. didn't dare B. not dared C. not dare D. dared not
解析，該題答案為A，此空需選一動(dòng)詞作謂語，因?yàn)楹竺媸莟o walk, didn't dare是行為動(dòng)詞dare過去時(shí)態(tài)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，該題答案為A, would此處表過去的傾向性，習(xí)慣性動(dòng)作，意為"總是"如: When we were children, we would go swimming every summer.
