She fell asleep while (she was) doing her homework. 她在做作业时睡着了。

　　He kept silent when (he was) asked why he was late. 问他为什么迟到，他一言不发。

　　Although (it was) built 100 years ago, the bridge is very strong. 虽然建于100年前，但这桥还是很牢固。

　　有些由if构成的省略结构，已属固定短语，如if any, if necessary, if possible, if not, if so等：

　　There are few, if any, mistakes in that book. 那本书就是有错误也不多。

　　If necessary, ring me at home. 如果必要，可以打电话到我家找我。

　　If possible, I wish to go there next summer. 如果可能，我希望明年夏天去。

　　He may be busy. If so, I’ll call later. If not, can I see him now? 他可能很忙，要是这样，我以后再来拜访。要是不忙，我现在可以见他吗?