英语基础语法19.6 条件状语从句

　　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.　
　 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.

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.