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.

例句：

Let's go out for a walk unless you are too tired.

If you are not too tied, let's go out for a walk.

例题：

You will be late ______ you leave immediately.

A. unless；B. until；C. if；D. or；

答案：A。

解析：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.