(1) 如果状语从句的主语与主句主语一致,而且状语从句谓语动词有be动词,这时可以将状语从句的主语连同be动词一起省略;引导这类状语从句的连词有:when, while, although, unless, if, as if / as though等。如:He came across the picture while (he is ) on a visit to New York.(他在去纽约时碰巧看到了那幅画。)Although (he was) frightened, he managed to run away.(虽然吓坏了,他还是跑掉了。)
(2) 在if it is possible, when it is necessary等类似结构中,it is可以省略。如:If possible, we’ll build another railroad in the area.(如果可能的话,我们要在这个地方再建一条铁路。)Don’t do it unless required.(除非需要,不然别这样做。)