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.





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