情態動詞
一、定義
情態動詞表示說話人對某一動作或狀態的態度。
二、特點
1．有一定詞義；
2．不受主語人稱和數的變化影響；
3．與主要動詞的原形(或稱不帶to的不定式)一起構成謂語(除ought to作固定片語看待)。
情態動詞無人稱和數的變化， 情態動詞後面跟的動詞需用原形，否定式構成是在情態動詞後面加 「not」。 個別情態動詞有現在式和過去式兩種形式， 過去式用來表達更加客氣， 委婉的語氣， 時態性不強， 可用於過去，現在或將來。情態動詞屬非及物動詞，故沒有被動語態。
基本助動詞與情態助動詞最主要的區別之一是，基本助動詞本身沒有詞義，而情態助動詞則有自己的詞義，能表示說話人對有關動作或狀態的看法，或表示主觀設想。
三、用法
1．can (could)
everyone here can speak english. (表能力)
he can't / couldn't h**e seen her there. (表猜測)
he could h**e gone home. (表猜測)
could / can i use your pen?
yes, of course you can. (表允許)
how can you be so careless? (表懷疑、驚異，主要用於否定、疑問句)
can / could you lend me a hand? (表委婉發表觀點)
2．may (might)
you may take whatever you like.(表允許，證據更委婉)
注：在回答以may引起的問句時，多避免用這個詞，而用其他方式，如yes, please. / certainly.
/ please don't. / you'd better not. / no, you mustn't.
等，以免顯得太嚴謹或不客氣。
they might be h**ing a meeting, but i'm not sure. (表可能)
3．must
you must buy a ticket. (表義務，意為「必須」)
注：回答must...? 引出的問句時，肯定回答用must; 否定回答不能用mustn't, 而要用needn't或don't h**e to。
—must i finish my homework now?
—yes, you must.(no, you needn't / don't h**e to.)
this must be lucy's. (表猜測，意為「想必；準是；一定」等，用於肯定)
「must h**e ＋ 過去分詞」表示「想必」之意，表示對過去的事情的推測。
4．shall
shall i play soccer after supper?
may i play soccer after supper?
if you don't beh**e yourself, you shall be punished.
5．will
i will do anything for you. 我願為你做任何事。 (表意願，用於各種人稱陳述句)
will you give me a cigarette? or i will tell the boss.(表請求，用於疑問句)
she will stand there sleeping for hours. (表示某種傾向或習慣性動作)
6．should
you shouldn't be so careless. (表義務，意為「應該」，用於各種人稱)
they should h**e arrived in beijing by this time. (表推測，意為「想必一定、照說應該、估計」等)
it is simply a miracle that rice should grow in such a place. (表示語氣較強的意為「假設；萬一；竟然」)
7．would
i said i would do anything for you. (表意願)
would you please tell me the way to the nearest bus stop? (表委婉地提出請求、建議或看法)
when he had a problem to solve, he would work at it until he found an answer. (表過去反**生的動作或過去的一種傾向)
8．ought to
i really ought to phone my mother.(表義務，意為「應該」，口氣比should稍重)
there's a fine sunset; it ought to be a fine day tomorrow. (表推測，暗含很大的可能，語氣較弱)
9．used to
there used to be a building at the street corner, but it has been pulled down. (表示過去的習慣動作或狀態，現在不再發生或不復存在)
i usedn't / didn't use to smoke. (否定式)
used you (did you use) to go to school on foot? (疑問式)
虛擬語氣
一、定義
虛擬語氣表示說話人的願望、假設、猜測或建議，而不表示客觀存在的事實。虛擬語氣通過謂語動詞的特殊形式來表示
二、用法
1．虛擬語氣用於非真實條件句中，一是在if條件狀語從句中；二是在含蓄虛擬條件句中，通過with, without, but for和動詞不定式短語等表示虛擬的條件或句中含有or, or else, otherwise, but等詞語，暗示句子的某一部分(前句或後句)需用虛擬語氣。。
if i were you, i would go with them. 假若我是你，我就同他們去。
if it hadn't been for your assistance, we wouldn't h**e succeeded. ＝ but for your assistance, we wouldn't h**e succeeded. ＝ without your assistance，we wouldn't h**e succeeded.
要不是你的幫助，我們就不會成功了。
if i asked him, i'm sure he'd help us.如果我向他提出要求，肯定他會幫助我們。
注意：錯綜時間虛擬條件句，即條件從句與主句所指時間不一致，如從句指過去，而主句即指的是現在或將來，此時應根據具體的語境情況，結合上面提到的三種基本型別對時態作相應的調整。
if it had rained last night, the ground would be wet now. 要是昨晚下過雨的話，現在地面就會是溼的。
you would be much better now if you had taken my advice. 假若你當時聽我的話，你現在就會好多了。
2．虛擬語氣用於主要是表示命令、要求、建議的動詞後的賓語從句中。常見動詞：乙個堅持：
insist；兩個命令：order, command；三個建議：advise, suggest, propose；五個要求：
demand, require, request, desire，ask。這些動詞後面的賓語從句要使用虛擬語氣用法。即從句中的動詞使用should ＋動詞原形，或者將should省略。
mr johnson insisted that the problem worthy of attention (should) be discussed at the meeting. 約翰森先生堅持應該
在會議外討論這個值得注意的問題。
he urged that they go to europe. 他敦促他們到歐洲去。
he suggested that we should le**e early. 他建議我們早點動身。
he ordered that it (should) be sent back. 他命令把它送回去。
注意：insist作「堅持說；強調」解時，賓語從句不用虛擬語氣；只有當insist作「堅持(應該)」解時，賓語從句才用虛擬語氣。
the stranger we caught insisted that he had never stolen anything. 我們抓住的那個陌生人堅持說他什麼東西也沒偷。
3．虛擬語氣用於wish後的賓語從句中，表示不能實現的願望，意為「可惜……；…… 就好了；悔不該……；但願……」等。表示現在不能實現的願望，從句的謂語動詞用過去式；表示將來不能實現的願望，用「would (could)＋動詞原形」；表示過去不能實現的願望，用「had ＋過去分詞」或「(could) would h**e＋過去分詞」。
she wished she had stayed at home. 她後悔的是她當時要是留在家裡就好了。(與過去相反)
i wish you would go with us tomorrow.要是你明天同我們一起去就好了。(與將來相反)
i wish i were you. 我要是你就好了。(與現在相反)
4．虛擬語氣用在would rather後的從句中。若與現在或將來事實不符，從句用一般過去時；若與過去事實不符，從句用過去完成時。
i would rather you paid me now. 我希望你現在就給我錢。(與現在事實相反)
i would rather you had attended the meeting yesterday. 我希望你昨天參加會議了。(與過去事實相反)
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