can
1.can用作“（罐头食品的）罐头”解是美国英语，相当于英国的tin。can用作“把…做成罐头食品”解原来也是美国英语，相当于英国的tin，但现在在英国也很普通。
2.cannot是英国拼写形式，can not是美国拼写形式。
3.许多语法书里说表示“许可”该用may，不该用can。其实用can表示“许可”也很普通，如You can go和You may go意思相同，Can I go?和May I go?意思也相同。You can...有时有“我要你…”的意思。
在口语里往往用cannot表示“不许可”。例如：Children, you cannot throw the ball over the wall.
4.请求他人作某事，说Could you ...?比说Can you ...?婉转些。
5.could（虚拟过去式）有时表示愿望或倾向，如I could eat another（我［现在］很想再吃一个）和I could cry for shame。（我［现在］几乎因为羞耻而哭出来了。）
6.can用在否定句和疑问句里往往表示可能，作“会…”解，如He cannot be over fifty（他不会在五十岁以上，他至多不过五十岁）和Can he be still living?（他仍旧活着吗？他现在还会活着吗？）can后面接着完成时态一定表示可能，如He cannot have seen her yesterday（他决不会在昨天见过她）和Can I have said so?（我会曾经这样说么？）注意：He cannot have seen her yesterday和He could not see her yesterday（他昨天不能见她）不同，Can I have said so?和Could I say so?（1.我曾经能够这样说么？2.我［现在］要这样说便能够这样说么？）不同。
7.You cannot be too careful直译作“你不会太小心”，就是说“你不论怎样小心，总不会过分”，也就是说“你愈小心愈好”。下面各句也都是这个意思：
- You cannot be over careful.
- You cannot be careful enough.
- You cannot be sufficiently careful.
- You cannot take enough care.
- You cannot take sufficient care.
下面各句都跟上面各句相仿：
- We cannot recommend this book too strongly.
- I cannot see enough of him.
- We cannot exaggerate his attainments.
- This point cannot be overemphasized.
8.can的过去式是could，说过去的“能够”用could当然不错。例如：I told him I could do it.但单说I could do it很容易被人看作有If I would do it, I could或I could do it, but somehow I don't do it的意思。所以指过去的“能够”通常不用could而用able。例如：I was able to do it.
比较下面各句：
- I told him I could do it.（指经常的或一次的能力，并不意味着实行）
- I was able to do it.（指经常的或一次的能力，并不意味着实行）
- I did it.（指经常的或一次的行为）
- I succeeded in doing it.（指一次的行为）
- I managed to do it.（指一次的行为）
“他最后精通了英语”不可说At last he could master English，通常也不说At last he was able to master English。而用下面的任何一句：
- At last he mastered English.
- At last he succeeded in mastering English.
- At last he managed to master English.
