【问】 He opened his mouth as if to speak.

as if是个从属连词，本应引导状语从句，它怎么会引出动词不定式呢?

【答】 在上面的句子里，as if仍然是个从属连词，它引导的是个省略型的从句：as if to speak=as if he wanted to speak。例如：

1. He shook his head as if (he wanted) to say “no”. (LDCE)

他摇了摇头，好像是(想)要说“不”。

2. She lifted one arm，as if to shield herself，then screamed in terror.

她抬起一只手臂，好象要护住自己，然后惊恐地尖叫起来。

当句子的主语与as if从句的主语相一致时，as if还可以引出现在分词和过去分词。在大多数情况下可以理解为省略了主语和be：

1. He paused as if expecting Lanny to speak. (Sinclair)

他停了一下，似乎在等待兰尼说话。

2. Parents were torn from their homes，husbands from their wives，to disappear forever asif swallowed up in death.

父母从家里被夺走，丈夫从妻子身边被掳去，犹如被死亡吞没一样，永远地消失了。

(参见as if expecting)