e.g. One cannot be too much careful. "越小心越好。"

二、cannot but + 动词原形，cannot help but + 动词原形，cannot help + 动名词，这三个短语在意义上基本相同，都表示"禁不住，不由得，不得不，不能不"的意思。cannot help + 动名词，是最规范的用法，最为常见。

e.g. I cannot help admiring the picture whenever I look at it. "不管什么时候看到这幅画，我都禁不住要赞赏它。"

"cannot but + 动词原形"主要用于美国英语，用法不大规范。

e.g. When the country calls you for help, you cannot but go. "当国家需要你的时候，你不能不挺身而出。"

"cannot help but + 动词原形"，这种用法多见于非正式文体。 

e.g. You cannot help but respect them. "你禁不住会尊敬他们。"

三、can but + 动词原形，cannot but + 动词原形意思不同，但又不能把后者简单地理解为前者的否定。can but + 动词原形 = can only。

e.g. I can but do my duty. "我只能尽我的职责。"

而cannot but + 动词原形 = must; to be forced to do something。

e.g. I cannot but go. "我必须走。"

四、"cannot have done"，"could not have done"都可以表示以现在的眼光看来，"过去还可能发生的事"。

e.g. They cannot/ could not have gone out because the light is on!

但could not have done 还可以表示从过去某个时间点看来，"以前还可能发生的事。

e.g. The girl asked to see a timetable, feeling sure that her father could not have made such a mistake.

另外，cannot have done 要比could not have done 语气要强一些。

还有一点值得注意：could not have done 有其相对应的肯定形式，即could have done。而cannot have done 却没有其相对应的肯定形式，即是说can have done 只能用在否定句和疑问句中。 

e.g.1) Where can he have gone?

2) She's two hours late -- What can have happened?

3) He can't have gone to Beijing --Just now I saw him.