1. 常用情态动词的基本用法
can , could 能，会
（could可以看作是can的过去式，这两个词除了时态不相同外，在表达意思的许多方面是相同的。当然could也有自己独特的用法。）
1）表示脑力或体力上的能力
Nobody can stop the development of science .
谁也无法阻止科学的发展。
She can sing that song in English.她能用英文唱那歌。
He could hardly support his family before he found the new job .他在找到那份新工作前几乎无 法养家。
I could not understand the lecture on the computer given by Mr. Smith .
我听不懂史密斯先生作的那个关于计算机的讲座。
2)表示“有时会”
This kind of situation can be very difficult. 这类情况有时会很困难。
3)表示允许，可以
Can I ask you some questions about it?
我可以问你有关这件事的几个问题吗？
You can not leave here till I come back .直到我回来你才能离开。
Such kind of thing can't happen any more later .
这类事以后不准再发生了。
Can you tell me how to get to the airport ? 你能告诉我怎么去机场吗？
Could you tell me how to get to the airport ?
您能告诉我怎么去机场吗？
Could I be forgiven my negligence ? 请原谅我的疏忽，行吗？
4）（用于否定句、疑问句或感叹句中）表示猜测、怀疑或惊诧
How can / could you be here ? 你怎么会在这儿？
She couldn't / can't be so stupid to do that .
她不可能蠢得去做那种事吧。
He couldn't / can't be over seventy .他不可能有七十多岁了。
Where could / can the boy be now ? 那孩子现在能在哪儿呢？
Could you speak a little slowly ?
您能稍微说慢一点儿吗？
I'm afraid that we couldn't give you any definite answer at this moment .
恐怕我们这时候还无法给予您任何确切的答复。
I could come earlier if asked .
如果要让我早一点来，我可以来早一点。
We would appreciate it if you could offer us any help .
如果您能为我们提供帮助的话，我们将不甚感激。
You could have done better if you had worked harder at it .
如果你再加把劲，本来可以干得更好一些的。
