时间、地点、条件、让步、方式状语从句等的主语与主句主语相同时，有时可用省略形式。如：
He fell asleep while (he was) doing his homework. 他在做着功课时睡著了。
Fill in a proper preposition where (it is) necessary. 在需要的地方填上适当的介词。
He will come if (he is) asked. 如果请他来，他会来的。
He won’t come unless (he is) invited. 他不会来，除非邀请他。
If (it is) necessary, ring me at home. 如果必要，可以打电话到我家找我。
He is better, though (he is) not yet cured. 他好一点了, 虽仍未痊愈。
It usually happens as (it is) described above. 情况通常就像上面描述的那样发生了。
He glanced about as if (he was) in search of something. 他向周围望了一眼，仿佛在找寻什么。
He cleared his throat as if (he was) to say something. 他清了清嗓子，像要说什么似的。
