1 情态动词的语法特征
1） 情态动词不能表示正在发生或已经发生的事情,只表示期待或估计某事的发生.
2） 情态动词 除ought 和have 外,后面只能接不带to 的不定式.
3） 情态动词没有人称,数的变化,即情态动词第三人称单数不加-s.
4） 情态动词没有非谓语形式,即没有不定式,分词,等形式.
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 have 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.
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.
4 比较have to和must
1) 两词都是'必须'的意思,have 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) have 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 have to 表示"不必"
mustn't 表示"禁止",
You don't have 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.
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 have worked hard all day.You 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 have been asleep.我刚才没有听到电话,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 have been sleeping,so I didn't hear it.
5) 否定推测用can't.
If Tom didn't leave 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.