21.2 比较can 和be able to
1）can could 表示能力；可能 (过去时用could),
只用于现在式和过去式(could)。be able to可以用于各种时态。
they will be able to tell you the news soon. 他很快就能告诉你消息了。
2）只用be able to
a. 位于助动词后。
b. 情态动词后。
c. 表示过去某时刻动作时。
d. 用于句首表示条件。
e. 表示成功地做了某事时，只能用was/were able to， 不能用could。
he was able to flee europe before the war broke out.
= he managed to flee europe before the war broke out.
注意：could不表示时态。
1）提出委婉的请求，（注意在回答中不可用could）。
---could i h**e the television on?
---yes, you can. /no, you can't.
2）在否定，疑问句中表示推测或怀疑。
he couldn't be a bad man.
他不大可能是坏人。
21.3 比较may和might
1） 表示允许或请求；表示没有把握的推测；may 放在句首，表示祝愿。
may god bless you!
he might be at home.
注意： might 表示推测时，不表示时态。只是可能性比may 小。
2） 成语： may/might as well，后面接不带to 的不定式，意为"不妨"。
if that is the case, we may as well try.
典型例题。peter __come with us tonight, but he isn't very sure yet.
a. must b. may c. can d. will
答案b. 表可能性只能用may. 此句意可从后半句推出。
21.4 比较h**e to和must
1) 两词都是'必须'的意思，h**e to 表示客观的需要， must 表示说话人主观上的看法，既主观上的必要。
my brother was very ill, so i had to call the doctor in the middle of the night. 我弟弟病得很厉害，我只得半夜里把医生请来。(客观上需要做这件事)
he said that they must work hard. 他说他们必须努力工作。（主观上要做这件事）
2) h**e to有人称、数、时态的变化，而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。
he had to look after his sister yesterday.
3） 在否定结构中： don't h**e to 表示"不必"
mustn't 表示"禁止"，you don't h**e to tell him about it. 你不一定要把此事告诉他。
you mustn't tell him about it. 你一定不要把这件事告诉他。
21.5 must表示推测
1) must用在肯定句中表示较有把握的推测，意为"一定"。
2) must表对现在的状态或现在正发生的事情的推测时， must 后面通常接系动词be 的原形或行为动词的进行式。
you h**e worked hard all must be tired. 你辛苦干一整天，一定累了。（对现在情况的推测判断）
he must be working in his office. 他一定在办公室工作呢。
比较： he must be staying there.
他现在肯定呆在那里。
he must stay there.
他必须呆在那。
3) must 表示对已发生的事情的推测时，must 要接完成式。
i didn't hear the phone. i must h**e been asleep. 我刚才没有听到**，我想必是睡着了。
4) must表示对过去某时正发生的事情的推测，must 后面要接不定式的完成进行式。
---why didn't you answer my phone call?
---well, i must h**e been sleeping, so i didn't hear it.
5) 否定推测用 can't。
if tom didn't le**e here until five o'clock, he can't be home yet. 如果汤姆五点才离开这儿，他此时一定还未到家。
21.6 表示推测的用法
can, could, may, might, must 皆可表示推测，其用法如下：
1）情态动词+动词原形。
表示对现在或将来的情况的推测，此时动词通常为系动词。
i don't know where she is, she may be in wuhan.
2）情态动词+动词现在进行时。
表示对现在或将来正在进行的情况进行推测。
at this moment, our teacher must be correcting our exam *****s.
这时，我们老师想必在批改试卷。
3）情态动词+动词完成时。
表示对过去情况的推测。
we would h**e finished this work by the end of next december.
明年十二月底前我们很可能已完成这项工作了。
the road is wet. it must h**e rained last night.
地是湿的，昨天晚上一定下雨了。
4）情态动词+动词的现在完成进行时。
表示对过去正在发生事情的推测。
your mother must h**e been looking for you.
你妈妈一定一直在找你。
5）推测的否定形式，疑问形式用can't, couldn't表示。
mike can't h**e found his car, for he came to work by bus this morning.
迈克一定还没有找回他的车，因为早上他是坐公共汽车来上班的。
注意：could, might表示推测时不表示时态，其推测的程度不如 can, may。
21.7 情态动词+ h**e +过去分词
1) may(might) h**e + done sth, can (could) h**e + done sth 表示过去，推测过去时间里可能发生的事情。
philip may (might) h**e been hurt seriously in the car accident.
philip can (could) h**e been hurt seriously in the car accident.
2) must h**e +done sth，对过去时间里可能发生的事情的推测，语气较强，具有"肯定"，"谅必"的意思。
---linda has gone to work, but her bicycle is still here.
---she must h**e gone by bus.
3) ought to h**e done sth, should h**e done sth
本应该做某事，而事实上并没有做。否定句表示"不该做某事而做了"。
you ought to (should) h**e been more careful in this experiment.
he ought not to h**e thrown the old clothes away.（事实上已扔了。）
ought to 在语气上比should 要强。
4) needn't h**e done sth 本没必要做某事。
i dressed very warmly for the trip, but i needn't h**e done so. the weather was hot.
5) would like to h**e done sth 本打算做某事
i would like to h**e read the article, but i was very busy then.
21.8 should 和ought to
should 和ought to 都为"应该"的意思，可用于各种人称。
---ought he to go?
---yes. i think he ought to.
表示要求，命令时，语气由 should（应该）、had better最好）、must（必须）渐强。
21.9 had better表示"最好"
had better 相当于一个助动词，它只有一种形式，它后面要跟动词原形。
had better do sth
had better not do sth
it is pretty cold. you'd better put on my coat.
she'd better not play with the dog.
had better h**e done sth表示与事实相反的结果，意为"本来最好"。
you had better h**e come earlier.
21.10 would rather表示"宁愿"
would rather do
would rather not do
would rather… than… 宁愿……而不愿。
还有would sooner, had rather, had sooner都表示"宁愿"、"宁可"的意思。
if i h**e a choice, i had sooner not continue my studies at this school.
i would rather stay here than go home. =i would stay here rather than go home.
典型例题。---shall we go skating or stay at home?
---which __do?
a. do you rather b. would you rather c. will you rather d. should you rather
答案b。本题考查情态动词rather的用法，would rather +do sth 意为"宁愿"，本题为疑问句， would 提前，所以选b。
21.11 will和would
注意： 1）would like； would like to do = want to 想要，为固定搭配。
would you like to go with me?
2）will you…? would you like…? 表示肯定含义的请求劝说时，疑问句中一般用some, 而不是any。
would you like some cake?
3）否定结构中用will，一般不用would, won't you是一种委婉语气。
won't you sit down?
21.12 情态动词的回答方式
问句肯定回答否定回答。
need you…? yes, i must. no,i needn't
must youdon't h**e to.
典型例题。1）--could i borrow your dictionary?
---yes, of course, you___
a. might b. will c. can d. should
答案表示委婉的语气，并不为时态。答语中of course，表示肯定的语气，允许某人做某事时，用can和 may来表达，不能用could或might。复习：
will 与you连用，用来提出要求或下命令。should与you 连用，用来提出劝告。
2）--shall i tell john about it?
---no, you __i've told him already.
a. needn't b. wouldn't c. mustn't d. shouldn't
答案a。needn't 不必，不用。 wouldn't 将不， 不会的。
mustn't 禁止、不能。 shouldn't 不应该。本题为不需要，不必的意思，应用needn't。
3）--don't forget to come to my birthday party tomorrow.
a. i don't b. i won't c. i can't d. i h**en't
答案b. will既可当作情态动词，表请求、建议、也可作为实义动词表"意愿、意志、决心"，本题表示决心，选b。
英语语法大全
英语语法大全范文。要想学好英语，就要先熟悉英语语法知识！只有熟练掌握英语语法，我们才不会把英语说成 中式英语 以下是是为大家的学语法的方法和常用语法汇总，欢送阅读！我国近年来的英语教学实践 在初学阶段，采用听说领先 学习根本句型的方法去学习英语语法，是行之有效的。句型学习是通过听说领先的方法去学习传...
英语语法大全
一 非谓语动词。非谓语动词 可分为动词不定式 动名词和分词。它在句子中的作用很多 除了不作谓语外，它可以充当主语 宾语 表语 定语 状语与复合宾语 主语补语或宾语补语 有些及物动词后面接不带to的不定式作复合宾语。这些动词归纳如下 一感 feel 二听 hear，listen to 三让 h e，1...
英语语法大全
目录。一 关于词类和句子成分 2 一 词类 parts of speech 2 1 名词 noun,缩写为n.2 2 代词 pronoun,缩写为pron。2 3 形容词 adjective，缩写为adj。2 4 动词 verb，缩写成v。3 5 数词 numeral，缩写成num。3 6 副词 a...