as if和as though引导的从句所叙述的情况与事实相反时，从句的谓语动词多用虚拟语气。在虚拟语气结构中，用________时态来表示现在的情形；用________时来表示过去的动作或状态；用________时表示正在进行的动作；用________时表示将要发生的动作。

当as if和as though从句的主语和主句主语一致，且从句中含有系动词be的某种形式时该主语和系动词可以省略。

例如：John paused as if(he was)expecting Mary to speak．

同位语从句与定语从句的区分

1)The proposal that he put forward is to be discussed at the meeting．

2)The proposal that we should hold sports meeting is to be discussed．

a．句1)是________从句，that在从句中________成分，句2)是________从句，that在从句中________成分，而且一般不省掉。

b．定语从句用来说明先行词的性质特征，起修饰或限制作用；同位语从句用来解释说明句词的内容，二者是________关系。

c．区分时可以在先行词与从句之间加一个系动词be，使之构成一个新句子，如果句子通顺且符合逻辑，则为同位语从句，反之，则为定语从句。

例如：The report that he was going to resign was false．他将辞职的传闻是假的。因为the report was that he was going to resign句意通顺，所以，that he was going to resign是同位语从句。