连接词主要有 if, unless, as/so long as, on condition that 等。if 引导的条件句有真实条件句和非真实条件句两种。非真实条件句已在虚拟语气中阐述。
unless = if not. 例如：
Let's go out for a walk unless you are too tired. 如果不太累，我们去散散步。
If you are not too tied, let's go out for a walk.
典型例题
You will be late ___ you leave immediately.
A. unless B. until C. if D. or
答案A。句意：除非你立即走，否则你就回迟到的。可转化为 If you dong leave immediately, you will be late。B、D句意不对，or表转折，句子如为 You leave immediately or you will be late.