疯豆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？
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？
