10could用法

1. could表示请求或允许，不限于疑问句，完全可以用作陈述句。下面的例句没有错，但最好把感叹号改为句号。因为情态动词过去形式是委婉礼貌的用于，说话时的口吻不会那么强烈。

You could/might sit here.

2.could/can表推测时：

can可以对现在和将来推测, 也可以表示对过去的推测。但都限于否定句和疑问句，不能用于肯定句。

That man can't be Mr. Smith. He has gone abroad. 表现在

If winter comes, can spring be far behind? 表将来

He can't have gone to the US. I saw him a few minutes ago. 表过去

can表示对过去的推测，can后接不定式完成体have done.

That could be my train 表现在（正确）

Tony could have been pretty naughty when he was a child. 表过去

理解正确。

3 could可以对过去，现在和将来进行推测, 可以用于肯定句、疑问句或否定句。表示现在对过去情况的推测时，要后接不定式完成体could have done.

I could go to Japan this summer 表将来（正确）

Could this have been done in those days? 这有可能是那时候做的吗？