英语四级关键英语语法知识：标准状语从句 连词关键有 if, unless, as/so long as, on condition that 等。. if 正确引导的条件句有真正条件句和非真正条件句二种。非真正条件句已在虚拟语气中论述。 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.
典型性练习题 You will be late ___ you leave immediately. A. unless B. until C. ifD. or 回答A。 句意：除非是你马上走，不然你也就回晚到的。可转换为 If you dong leave immediately, you will be late.B、D句意不对，or表转折点，语句若为 You leave immediately or you will be late.