助動詞本身無意義，在句中幫助主要動詞構成一定的時態，語態、語氣，或是幫助構成否定句和疑問句，常用的助動詞有be, do, have, shall(should), will(would)。

　　情態動詞表一定的詞義，本身并不表示動作或狀態，而僅僅表達說話人的態度，它在句中須和主要動詞一起構成謂語， 主要的情態動詞有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.

　　提建議或請求時可用can I, can you表客氣，如Can I buy you a drink?

　　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這二詞有實意動詞和情態動詞兩種詞性，如用作實意動詞后接動詞不定式to do，如用作情態動詞后接動詞原形。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對方請求，如，Shall 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表應該，意為有責任，有義務。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表應該做而未做

　　must have done表對過去事實的肯定推測

　　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.

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