情态动词表示说话人对动作或状态的各种观点和态度，如需要、猜测、意愿或怀疑等。情态动词有词义，但不完全，是所谓的“辅助性”动词，在句中不能单独充当谓语。
一、 情态动词的特征和形式
a. 情态动词的各种形式见下表：
情态动词
may
might
can
could
must
have to
ought to
will
would
shall
should
need
dare
used to
否定式
may not
might not
cannotcan not
could not
must not
do not have to
ought not to
will not
would not
shall not
should not
need not
dare not
used not to
did not use to
简略否定式
mayn't（老式英语，现在不常见）
mightn't
can't
couldn't
mustn't
don't have to
oughtn't to （否定句中to可省略）
won't
wouldn't
shan't （只用于英国英语）
shouldn't
needn't
daren't
usedn't to
didn't use to
b.情态动词除ought to, used to等外，后面只接不带to的不定式。
1.情态动词+do
you shouldn't be so careless. 你不该这样粗心大意。
jessica told him yesterday she might not go on the trip.
杰西卡昨天告诉他，她可能不去旅行了。
difficulties can and must be overcome.
困难能够而且必须克服。
2.情态动词+be doing
she must be listening to pop music.
她肯定在听流行音乐。
you should be reviewing your lessons.
你应该在复习功课。
my mother maymight be cooking now.
我妈妈可能正在做饭。
3.情态动词+have done
they might have visited the great wall.
他们可能参观过长城了。
he must have got up very early to catch the train.
他一定起得很早去赶火车了。
you ought to have come earlier.
你本该早一点儿来。
4.情态动词+be done
this word can also be used as a verb.
这个词也可以用作动词。
something must be done to stop pollution.
必须采取措施来制止污染。
the work ought to have been finished long ago.
这工作早就该完成了。
c. 情态动词第三人称单数一般现在时没有词形变化。
you ought to wear a raincoat.
你应该穿件雨衣。
she ought to wear a raincoat.
她应该穿件雨衣。
plants must have oxygen in order to live.
为了存活植物必须有氧气。
a plant must have oxygen in order to live.
为了存活植物必须有氧气。
d. 情态动词的时态并不是区分时间的主要标志。在一些场合中，情态动词的现在式和过去式都可以表示现在时间、过去时间或将来时间。
i'm afraid it might rain tonight.
我看今晚可能要下雨。
could i borrow your thermos
我可以借用你的暖水瓶吗？
it's a nice day today. we could go for a walk.
今天天气不错，我们可以出去散散步。
e. 情态动词是互相排斥的，一般不允许两个意义相近的情态动词连用。
【误】soldiers must have to obey orders.
【正】soldiers have to obey orders.
军人必须服从命令。
【正】soldiers must obey orders.
军人必须服从命令。
【误】can i be able to borrow two books at a time
【正】can i borrow two books at a time
我能一次借两本书吗？
【正】will i be able to borrow two books at a time
我能一次借两本书吗？
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