一、由when/while/as soon as/once/until/if/unless/where等引导的状语从句，当从句主语与主句主语一致或从句主语为it时，可省去主语和be。
I’ll not go unless (I am) invited.
The news is important, if (it is) true.
注意，after/before没有这种用法。
二、由though/although/however/whatever/no matter how/what等引导的状语从句可省略主语+be或be。
He works very hard though (he is) still rather weak.
In our country everyone is an ordinary worker, no matter what his position (is).
在我国人人都是普通劳动者，不管他地位如何。（仅供举例用，无关内容真实与否。）
