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?

　　另外，could还常用于表示较客气委婉的看法、提问及用于虚拟语气的结构中。这时候，could就不可以看作是can的过去式了。而是could自己独特的用法。如：

　　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。