为了简洁起见，有的状语从句(如时间状语从句、条件状语从句、方式状语从句等)有时可省略从句的主语和部分谓语(尤其是当从句主语与主句主语一致，且从句谓语包括有动词be时)：

She fell asleep while (he was) doing his homework. 他在做作业时睡着了。

While (he was) doing so, he trembled a little. 他这样做时稍稍颤抖了一下。

He opened his mouth as if (he was) to speak. 他张开口，好像要说话似的。

Often she would weep when (she was) alone. 她一个人时，常常哭泣。

I'd like to see you whenever (it's) convenient. 在你方便的时候我想来看看你。

If (you are) traveling north, you must change at Leeds.
如果你是向北行，你必须在里兹换车。

【注】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：

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?