自從今年初開始申請實習開始，需要商業英文書信往來的機會增加了許多，最近常看到這種句型：
Should you have any questions please don't hesitate to ask.
這個句型特別的是把Should放在句首，但其實是If you should have any questions...的變化而來，
可是這樣的句子，跟If you have any questions...(這也是我最常用在書信的方式)到底語意有何不同？
上網google了一下，發現有些老外朋友也有相同的疑問，在此節錄一些人討論的結果：
It's just another way of expressing the second (or third) conditional by the means of auxiliary-subject inversion.
More examples
If I were to find a treasure I'd buy a castle.
--> Were I to find a treasure, .....
If he hadn't been there, we would have lost the game
---> Had he not been there, we.....
In both sentences we have inverted the subject and the auxiliary (or modal). Plus we no longer need the if.
It's both more idiomatic (in the sense that this wording is specific to English) and slightly more formal.
"Should" is often used in a business context like this because to some people, it sounds more formal. In short, it replaces "if."
There is also a subtle idea about "should" in that it is supposed to express a minimal amount of doubt.
If you have any questions (I don't know if you will or not), please don't hesitate to ask.
Should you have any questions (and I rather doubt you will), please don't hesitate to ask.
