If you should do/If you do

if + 动词现在时, 是规范的英语真实条件从句。同样, if + should + 动词不定式, 也是规范的真实条件从句。两者的区别, 仅仅在于后者增加了条件的不确定性。

(A) If you are interested, I will send you a copy of my book. 如果你感兴趣,我将把我的书送给你一本。

(B) If you should be interested, I will send you a copy of my book. 要是你感兴趣的话，我将把我的书送给你一本。

2. $\$int(A) If you leave him, he will kill himself. 如果你离开他,他会自杀的。

3. $\$left\{ (B) If you should leave him, he will kill himself. \right. 要是你离开他的话，他会自杀的。

(A) If I see Mr. Allen, I'll ask him to ring you. 如果我见到艾伦先生，我叫他给你打电话。

3. $\$left\{ \begin{array}{l}\text{(B)If I should see Mr. Allen, I'll ask him to ring you. 要是我能见到艾伦先生的话,我叫他给你打电话。} \end{array} \right.

在句(B)中,如果主句是一个祈使句,如你所提出的试题那样,常常用于表达礼貌地请求或建议:

1. If he should call, tell him I'll ring back. (LED) 要是他来电话,请告诉他我将给他回电话。

2. If you should be passing, do come and see us. (Eckersley) 要是你路过这儿的话, 就来看我们吧。

3. If you should see Celia, give her my best wishes. (PEU) 要是你见到西莉亚,请代我问候她。

4. Should there be a fire, walk, don't run, to the nearest exit. 一旦着火, 不要奔跑, 要走到最近的安全门去。