怎么样讲解If you would be so ki

I should be grateful if you would be so kind as to allow Jim to leave school on Friday. 你若是能准吉姆星期五上去上学的假，我将不胜感激。
请问这里的 if 从句为何用 would be 而不需要 were 呢？

依据《牛津英语使用方法指南》的看法：……在有礼貌的请求中，通常不需要 were, 而用 would be。如：

We should be grateful if you would be so kind as to let us have your cheque as soon as possible. 如蒙尽快将支票交来，大家将不胜感激。

We would appreciate it if you would be so kind as to let us have your cheque by return.如蒙即送支票，大家将不胜感激。