4．表示惊异、怀疑、不相信等态度，主要用于否定句和疑问句中。
How can
you be so careless
你怎么这么粗心？
Where
can could they have gone
他们会去了哪里？
He
can'tcouldn't be over sixty.
他不可能超过60岁。
5．Could可以用在虚拟条件句中。
If I
could fly, I should be very happy.
如果我能飞，我会很高兴。
If you
had followed my advice, you could have finished it.
如果你当时听我的，你早就做完了。
If you
had tried harder, you could have passed your exam.
如果你再努力些，你就能通过考试了。
６．can与be
able to的区别
①can表示“能力”时，和be able to相当，许多场合都可以互相替换。但当叙述过去经过一番努力才能完成的事情或前面有特殊说明，表示你有能力时，只能用be
able to。
【正】Can you speak any foreign languages
你会说外语吗？
【正】Are you able to speak any foreign languages
你会说外语吗？
【误】The fire spread the building quickly but
everybody could escape.
【正】The fire spread the building
quickly but everybody was able to escape.
大火迅速蔓延到整幢大楼，但大家都逃了出来。
【正】The fire spread the building quickly but
everybody managed to escape.
大火迅速蔓延到整幢大楼，但大家都想法逃了出来。
②be able to比can有更多形式。
No one
could answer the question.
没人能回答这个问题。（这里could可用was
able to代替）
When he
grows up, he will be able to support his family.
他长大后就能养家了。
Frank is
ill. He hasn't been able to go to school for one week.
弗兰克病了，已经一周没去上学了。
I'm
sorry for not being able to help you in time.
对不起，不能及时帮你的忙。
③could经常和动词see,
hear, smell, taste, feel, remember, understand等连用。
When we
went into the house, we could smell something burning.
当我们走进屋子时，我们闻到什么东西烧焦了。（不用was
able to）
She
spoke in a very low voice, but I could understand what she said.
虽然她讲话的声音很低，但我还是明白了她说的话。
④在谈论说话时发生的动作，用can，不用be
able to。
【误】Look!
I'm able to swim.
【正】Look!
I can swim.
看，我会游泳了！
C．must和have
to
1．must表示有做某一动作的必要或义务，强调主观看法，可译为“必须，应该”。
Everyone
must obey the rule.
人人都必须遵守制度。
We must
do everything step by step.
我们必须逐步做每件事。
Teachers
must be patient enough with their students.
老师必须对学生有足够的耐心。
2．have
to表示因客观需要促使主语不得不做某事。
You
can't turn right here. You have to turn left.
你不能在这里右拐，你必须左拐。
My
eyesight is very poor. I have to wear glasses for reading.
我的视力很差，看书时不得不戴眼镜。
You have
to pass a test before you can get a driving licence.
你在取得驾照之前必须先通过考试。
提示：
在口语中，我们可以使用have
got to来代替have to.
I've got
to work on Sunday.
我不得不星期天工作。
When has
Ann got to go
安必须什么时候走？
3．must与have
to的比较。
must与have
to在表示“必须”这个意思时意义相近，但在用法上有所区别。
①must含有说话者的强烈决意（表示主观的看法），have
to则表示外力环境或习惯使然（即表示客观的必要，作“不得不”解）。
I must
study hard.
我必须努力学习。（发自内心的决定）
I have
to study hard.
我不得不努力学习。（外界因素逼迫）
You must
stay for supper.
你必须留下来吃晚饭。（because
I want you to）
You have
to stay for supper.
你得留下来吃晚饭。（because
there is nowhere else to go）
②must只有一种形式，而have
to可有多种时态。
They had
to put off the sports meet due to the bad weather.
由于坏天气，他们不得不推迟运动会。
These
last two days he has had to take a rest at home.
这两天他必须一直在家休息。
③must的否定式和have
to的否定式的意思完全不同。
You must
keep it a secret. You mustn't tell anyone.
你必须保密，你不可以告诉任何人。（mustn't表示“禁止”）
You
don't have to tell me the secret.你不必告诉我这个秘密。
（don't have to意思是“没有必要”= don't need to）
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