连接词主要有 if, unless, as/so long as, on condition that 等。.
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.
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 don't 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.