表示允许、许可，常用在口语中。 could 比 can 语气上要客气。

 我可以用一下你的字典吗？ Could I use your dictionary?

 可以，用吧。 Yes, go ahead

  Yes, you can

不能说 Yes, you could

劳驾，你能告诉我怎么去动物园吗？Could you tell me how to get to the zoo?   

  Can you tell me how to get to the zoo?

 他问我可不可以把书带出阅览室。He asked me whether he could take the book out of the reading room

 can 表示推测，常见于疑问句或否定句。

 can 表示推测的可能性比 could 大。

can 表示推测很少用于肯定句

 他会是生病在家吗？Can he be ill at home?

 这个故事会是真的吗？Can the story be true?

 他不可能在家。He cannot be at home

 在林子里走时一定不要吸烟，那样可能会引起火灾。You mustn't smoke while you are walking around in the woods, You could start a fire

 表示惊异、怀疑、不相信等态度，主要用于否定句和疑问句中。

你怎么这么粗心？How can you be so careless ?

他们会去了哪里？Where can they have gone ?

  Where could they have gone ?

 他不可能超过60岁。He can't be over sixty  

 He couldn't be over sixty   

 Could 可以用在虚拟条件句中。

 如果我能飞，我会很高兴。If I could fly I should be very happy

 如果你当时听我的，你早就做完了。If you had followed my advice you could have finished it

 如果你再努力些，你就能通过考试了。If you had tried harder you could have passed your exam

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 。

 你会说外语吗？Can you speak any foreign languages？

 你会说外语吗？Are you able to speak any foreign languages？

 大火迅速蔓延到整幢大楼，但大家都逃了出来。The fire spread the building quickly but everybody was able to escape

 大火迅速蔓延到整幢大楼，但大家都想法逃了出来。The fire spread the building quickly but everybody managed to escape

 没人能回答这个问题。No one could answer the question

这里 could 可用 was able to 代替

 他长大后就能养家了。When he grows up he will be able to support his family

弗兰克病了，已经一周没去上学了。Frank is ill,  He hasn't been able to go to school for one week

 对不起，不能及时帮你的忙。I'm sorry for not being able to help you in time

could 经常和动词 看，see， 听，hear，闻， smell， 尝，taste，感觉， feel， 记住，remember， 理解，understand ，等连用。不用 was able to

 当我们走进屋子时，我们闻到什么东西烧焦了。When we went into the house we could smell something burning

 虽然她讲话的声音很低，但我还是明白了她说的话。She spoke in a very low voice, but I could understand what she said

在谈论说话时发生的动作，用 can ，不用 be able to 。

 看，我会游泳了！Look ! I can swim

 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”。

 人人都必须遵守制度。Everyone must obey the rule

 我们必须逐步做每件事。We must do everything step by step

 老师必须对学生有足够的耐心。Teachers must be patient enough with their students

 have to 表示因客观需要促使主语不得不做某事。

 你不能在这里右拐，你必须左拐。You can't turn right here,  You have to turn left

 我的视力很差，看书时不得不戴眼镜。My eyesight is very poor,  I have to wear glasses for reading

 你在取得驾照之前必须先通过考试。You have to pass a test before you can get a driving licence

在口语中，我们可以使用 have got to 来代替 have to

 我不得不星期天工作。I've got to work on Sunday

安必须什么时候走？When has Ann got to go ?

must 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）。

 我必须努力学习。I must study hard

 我不得不努力学习。I have to study hard

 你必须留下来吃晚饭。You must stay for supper

因为我想让你，because I want you to

 你得留下来吃晚饭。You have to stay for supper

 因为没有别的地方可以去because there is nowhere else to go

must 只有一种形式，而 have to 可有多种时态。

 由于坏天气，他们不得不推迟运动会。They had to put off the sports meet due to the bad weather

 这两天他必须一直在家休息。These last two days he has had to take a rest at home

 你必须保密，你不可以告诉任何人。 You must keep it a secret, You mustn't tell anyone

mustn't 表示“禁止”

 你不必告诉我这个秘密。You don't have to tell me the secret

没有必要don't have to,  

  don't need to