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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