情态动词can与could的用法归纳
1. 表示能力
(1) 表示现在的能力，用can，如：
He can (=He is able to) swim. 他会游泳。
He can speak English well. 他英语能说得很好。
I can’t remember where I put it. 我记不得把它放在那儿了。
(2) 表示将来的能力，通常用will / shall be able to，如：
I shall be able to come tomorrow. 明天我能来。
One day people will be able to go to the moon on holiday. 总有一天人们可以到月球上去度假。
但是，若表示现在决定将来是否有能力做某事，则可用 can，如：
Can you come to the party tonight？你今晚能来参加我们的聚会吗？
(3) 表示过去的能力。
① 若泛指过去的能力，可用could，如：
I could run faster then. 我那时能跑得更快一些。
②若表示过去特定场合做某事的能力，则不能用could，而用was / were able to do sth.，或用 managed to do sth，或用 succeeded in doing sth 等。如：
He studied hard and was able to pass the exam. 他学习很努力，所以考试能及格。
= He studied hard and managed to pass the exam.
= He studied hard and succeeded in passing the exam.
误：She could jump into the sea before the ship was blown up. 船爆炸之前，她跳进了大海。
正：She was able to jump into the sea before the ship was blown up.
注：在下列情况，could也可表示过去特定的能力：
①在否定句中，could和was / were able to可通用。如：
I couldn’t / wasn’t able to see him before he left. 他离开之前我没有再见到他。
②could与see, hear, smell, taste, feel, understand等表示感知的动词连用时。如：
I can hear you easily from here. 我在这儿就能很轻易地听到你的话。
Then he spoke slowly, but I could not understand him. 接着他说得慢了,但是我听不懂他的话。
③could用在间接引语中时。如：
He said he could finish the work next week. 他说他下周能完成工作。
顺便说一下，be able to还可用于完成时, 如：
I’m sorry I haven’t been able to come earlier. 对不起，我没有能够来得更早一些。
He said he had not been able to answer your letter. 他说他一直没能给你回信。
2. 表示许可
(1) 对于现在或将来的许可，用can或could均可，但could 语气更委婉, 如：
Can / Could I smoke in here? 我可以在这儿吸烟吗?
You can't pick the ball up in football. 踢足球时不许可用手持球。
I could come tomorrow if you like. 如果你愿意，明天我可以来。
② 表示自己允许别人做某事时, 答语应用can或may，而不用 could。如：
--Could / Can I use your pen? 我可以借用你的钢笔吗?
误：-- Yes, of course you could. 当然可以。
正：--Yes, of course you can / may.
(2) 对于过去的许可。 若表示过去一般性允许用could；表示过去特定的允许，则不用 could，而需换成其它表达，如had permission, was / were allowed to。如：
When I lived at home, I could watch TV whenever I wanted to. 我住在家里时, 想什么时候看电视就什么时候看。(一般性允许)
I was allowed to see the film yesterday evening. 昨天晚上允许我去看了电影。(特定的允许)
3. 表示推测
(1) 用can或could+动词的一般式，表示对现在或将来的推测。
①can通常用于否定句或疑问句中，而could则可用于肯定句、否定句或疑问句。如：
Even experienced teachers can make mistakes. 即使是有经验的教师也可能出错。
The wheel of history can not be turned back.历史的车轮决不会倒转。
I could go Beijing this summer. 今年夏天我们可能要去北京。(将来可能性)
You could be right, but I don’t think so. 你可能是对的，但我认为不是。(现在可能性)
(2) 用can / could +动词的进行式，对正在发生的情况进行推测。如：
What can / could he be doing at home now? 他现在在家可能在干什么呢？
He can’t be watching TV all day. 它不可能一整天都在看电视。
(3) 用can / could +动词的完成式。表示对过去的推测。如：
Alice can’t have written an article like that. Alice不可能写出那样的文章。
He can’t / couldn’t have arrived already. 他不可能已经到那儿。
He could have left. 他可能已离开了。
Where can / could he have gone? 他能到那儿去了呢？
还可表示“本来能够干某事”但事实并非如此，如：
He could have passed the driving test. 他本来能够通过驾驶考试的。(但事实没有通过)
You can / could have done it better. 你本来应该做得更好。
4. can还可表示惊讶，怀疑，不相信的态度
主要用于否定句、疑问句或惊叹句中，如：
Can this be true? 可能是真的吗？
How can you be so careless！你怎能如此粗心！
This cannot be done by him. 这不可能是他做的。
5. 常见的固定搭配
(1)can not help doing= can not but do 意为“不能不，禁不住”。如：
I cannot help laughing. = I cannot but laugh. 我不禁笑了出来。
I can’t help being what he be. 他成为这样的人物身不由己。
(2) cannot do / be too… 为“无论……也不过分”。如：
You cannot attach too much important to this matter. 你应该把这件事视为万分重要。
We can’t b too careful in driving a car. 我们无论如何谨慎驾驶也不过分。
We can’t praise him too much. 我们无论怎样表扬他都不过分。
(3) as… as one can / could 意为“尽可能地”，如：
I will come as soon as I can. 我尽可能早来。
I ran as fast as I could. 我尽快地跑。