1. 条件状语从句（Adverbial Clauses of Condition）主要由if或unless引导：

If I had been less cautious，I might have been more wise. 要是我不那么谨慎，也许我还明智一点。

We can't exercise our influence unless we have an organ.

2. 条件状语从句还可以由其他连词或起连词作用的短语引导：

Supposing（that）you are wrong，what will you do？假定你是错的，你怎么办？

Suppose you had a million pounds，how would you spend it?

3. 由if引导的条件状语从句，有时可把if省略，而把从句的语序倒装：

Were it not for their assistance，we would be in serious difficulty. 如果不是他们帮忙，我们会陷入严重的困境。