3. whether和if的意思为"是否"，一般情况下，二者都可以引导宾语从句；但是在介词之后的宾语从句通常只能用whether引导；另外，从句后有"or not"或后接动词不定式时，也只能用whether引导。

David asked her if/whether she slept well last night. 大卫问她昨天晚上是否睡得好。

They are worrying about whether they can get there in time. 他们正在担心是否能及时到达那儿。

I can't decide whether to stay. 我不能决定是否要留下。

　　4. 如果宾语从句后还有宾语补足语，则要用it作形式宾语，而将宾语从句后置。另外appreciate, love, like, dislike, hate等动词由于其后的宾语只能为"物"，不能为"人"，其后不直接跟if或when引导的从句，若语义上需要接这类从句，需借助it作形式宾语。

He has made it clear that he will not give in. 他清楚地表明他不会屈服。

I would appreciate it very much if you would help me with it. 如果你能帮助我做这件事,我将不胜感激。

二、表语从句

　　1. 引导表语从句的词:

连接词：that, whether, because, as if

连接代词：who, whom, whose, what, which, whatever, whoever ...

连接副词：where, when, how, why

That's just what I want. 这正是我想要的。

This is where our problem lies. 这就是我们的问题所在。

It looks as if it is going to rain. 看上去要下雨了。

　　2. that在引导表语从句时无词义，也不充当任何成分，但是通常不省略。

The fact is that we have lost the game. 事实是我们已经输了这场比赛。

　　3. 在引导表语从句时通常只能使用whether，意为"是否"，不能使用if。

The question is whether she can arrive at the hotel on time. 问题是她是否能按时到达旅馆。

　　4. 主语是reason时，表语从句要用that引导，而不是because或why。

The reason why he was late was that he got caught in a traffic jam on the way.

他迟到的原因是在路上遇到了交通堵塞。

表语从句是在复合句中充当表语的从句，通常放在句子的系动词后面。引导表语从句的关联词有三类：从属连词that，从属连词whether，连接代词who, whom, whose, what, whic