情态动词。
情态动词表示说话人对某一动作或状态的态度，可以表示“可能”、“可以”、“需要”、“必须”或“应当”等之意。情态动词没有人称和数的变化。但不能单独作谓语动词用，必须和不带to的不定式连用构成谓语动词。
只有情态动词ought要和带to的动词不定式连用，在句中作谓语用。
一． can/ be able to
can 表客观的能力，be able to表通过主观的努力，能够……
eg. he is a native speaker of english, so he can speak english well.
the fire was big, but he was able to escape.
二．shall 用于。
二、三人称表命令，警告，威胁。宣布法律，法规时用shall.
eg. the judge declared that all the money shall belong to the wife.
—what does the sign over there read?
—“no personsmoke or carry a lighted cigarette，cigar or pipe in this area.”
三．1. should 表推测时指确定或期望，即合乎理想的情况或结果。
it’s nearly seven o’clock. jack should be here at any moment.
2. should 表“竟然”
how should he do like this?
3. must 用于条件句或疑问句中，可以用来表示责备，抱怨的感**彩，意为“硬要，偏要，干嘛'
eg. if you must, choose a seat in the smoking section.
四．1. may 表征求对方意见。
may i h**e a look at it ?
2． 表推测时may 表不确定。
eg. the train might le**e at six, but i’m not sure.
3. may /might as well 不妨……，不如……
eg. it is dark and you may as well stay here .
since the flight was cancelled， you might as well go by train.
五。 情态动词后加动词原形表对目前的推测，后加h**e done 表示对过去的推测。
1. must，may, might 用于肯定句，语气减弱。
can’t, couldn’t 意为“不可能”，用于否定句，疑问句。
但不存在mustn’t h**e done
eg. he must be at home.
he may go out .
he can’t be watching tv.
he couldn’t h**e finished the job yesterday, for he was badly ill.
6．情态动词+ h**e done 有两层含义：
（一）表对过去的推测。
1. must h**e done 表示对过去已经发生的行为进行推测，意为“一定做了某事”
2. could h**e done 本可以……（实际没有）
he could h**e been admitted to the college, but he failed.
/ might h**e +过去分词，表示对已发生的事情做不肯定、可能性很小的推测，或事实上根本没发生，译为“也许……”
4. couldn’t h**e done / can’t h**e done 表示对已发生情况的否定推测，不可能做了某事”
was limited, so he couldn’t h**e finished the task.
marymy letter; otherwise she would h**e replied before now.
a] couldn’t h**e received [b] ought to h**e received
c] has receivedd] shouldn’t h**e received
二）表虚拟。
1. should (not) h**e done
ought (not) to h**e done 本（不）应该做某事，但实际上没做（做了）。
eg. you should h**e come here earlier.
you oughtn’t to h**e eaten too much.
2. needn’t h**e done “本没有必要做某事，但实际上已经做了”
eg. you needn’t h**e taken a taxi here, for it was near to your house.
needn’t do “没必要做某事”
you needn’t go now.
as it turned out to be a small house party， weso formally.
a] needn’t dress upb]did not need h**e dressed up
c] did not need dress up [d] needn’t h**e dressed up
七．can’t be too can’t never be too “再怎样也不过分”
eg. you cannot be too careful when you drive a car.
8．it couldn’t be better. /it couln’t be worse. (交际用语)
比较级与否定连用，表示最高级。
how do you find the restaurant?
it couldn’t be better. /it couln’t be worse.
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