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

情態(tài)動詞表一定的詞義，本身并不表示動作或狀態(tài)，而僅僅表達說話人的態(tài)度，它在句中須和主要動詞一起構(gòu)成謂語，?主要的情態(tài)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(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這二詞有實意動詞和情態(tài)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(tài)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?qū)Ψ秸埱?，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(yīng)該，意為有責任，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.?