楼上俩同学基本是“学术派”传人,分析得活灵活现.
我可以告诉你,从实用的角度而言,
它们基本没有区别,除此之外教科书上应该还学过
If it should rain ...
Should it rain ...
这两种也一样是虚拟语气的表达.
但真正在口语时,类似这种情况的对话绝大多数都会力求简单,采用第一种.
不过下列这种虚拟句在口语中就常用到,因为无可替代：
If I were you,I would ...
If I had known,I would/could have ...
商业文书中常见的 If you should have any question,please feel free to call me at .
这类句子真的也只在书面文字使用,口语时很少有人这么啰嗦的.