语法专题四 名词性从句
在句子中起名词作用的句子叫名词性从句。 名词性从句的功能相当于名词词组，它在复合句中能担任主语、宾语、表语、同位语等，因此根据它在句中不同的语法功能，名词性从句又可分别称为主语从句、宾语从句、表语从句和同位语从句。
考点一 whether 与if
whether与if 在作“是否”的意思讲时在下列情况下一般只能用whether，不能用if：
1．引导主语从句并在句首时。
Whether the meeting will be held tomorrow has not been decided yet.
2．引导表语从句和同位语从句时。
We should discuss carefully the question whether we can finish it on time.
3．引导从句作介词宾语时。
It all depends on whether they will lend us the money.
4．从句后有“or not”时。
I didn't know whether or not he had arrived in Beijing.
5．后接动词不定式时。
I don't know whether to go to the party.
考点二 that，what与which
1．that 引导名词性从句时，在从句中不充当任何句子成分，只起连接作用，也没有任何含义；that 引导的宾语从句一般不作介词的宾语(介词but，except，besides，in除外)。
That he failed the exam made me surprised.(主语从句，that不充当从句成分)
He is a good student except that he is a little careless.
2．what引导名词性从句时，其意义为“……的人/物/数目等”，在从句中充当主语、宾语、表语或定
