3．当句子是以it作形式宾语时，引导宾语从句的that不可省略。如：
he has made it clear that he will not give in.
4．that一般不能引导介词的宾语从句，偶尔可在except, but, besides和in（在于…；在…方面）的后面引导宾语从句，that不可省略。如：
your homework is quite good except that you h**e made two small mistakes.
the reason lies in that she works harder than the others do.
he differed from other people in that he always looked farther ahead in his work.
在其它介词后面需要用that引导宾语从句时，必须用it作形式宾语。如：
you may depend on it that i shall always help you.
5．在表语从句和同位语从句中，that不可省略。如：
my advice is that you must give up smoking right now.
the fact that he is a thief shocked all of us.
二、以whether/if引导从句。
引导名词性从句的whether/if意为“是否”。在使用时，要注意两者存在以下区别。
1．在主语从句中，whether既可引导位于句首的主语从句，也可引导用it作形式主语的主语从句，而if只能引导以it作形式主语的主语从句。如：
whether the work can be completed on time is doubtful.
it is doubtful whether/if the work can be completed on time.
2．在宾语从句中，当其后紧跟or not时，只能用whether，不能用if。如：
i don’t care whether/if he comes or not.
i don’t care whether or not he comes.
3．在介词后面的宾语从句中，只能用whether，不能用if。如：
it all depends on whether they will come back.
4．在表语从句和同位语从句中，只能用whether，不能用if。如：
the problem is whether the money will be enough.
i h**e no idea whether the money will be enough.
三、以特殊疑问词引导从句。
英语中所有用来构成特殊疑问句的特殊疑问词均可以用来引导名词性从句，其选择取决于从句句意的需要。如：
it remained unknown who（谁） let out the news.
i don’t know what（什么）i was thinking of.
the problem is where（**） we should stay.
my question how（怎样） i shall get in touch with him has not been answered.
选择特殊疑问词需注意的情况：
1．要注意特殊疑问词在从句中所担当的句子成分：疑问代词可作主语、宾语和表语，而疑问副词（where, when, why, how）只能作状语。如：
i can’t imagine who did it. （作主语）
i can’t imagine how he did it. （作状语）
2．who在从句中既能作主语，也能作宾语，而whom只能作宾语。如：
it hasn’t been decided who will manage the company.
they h**en’t decided whom they should help.
四、以关系词引导从句。
用关系词引导的名词性从句又可称作名词性关系从句，因为它实际上是先行词与在其后的定语从句的结合。引导这种名词性从句的关系词有：what(=the thing that), whoever(=anyone who), whichever(=anyone/anything that), where(=the place where), when(=the time when)等。
如：what they need is a good rest.
i can judge by what i know of him.
whoever did this job must be rewarded.
whichever he likes will be given to him.
we shall not forget when the meeting will be held.
she walked up to where he stood.
用关系词引导名词性从句时需要注意wh-ever与no matter wh-的区别。wh-ever既可引导名词性从句，又可引导让步状语从句；而no matter wh-只能引导让步状语从句。如：
whatever i said, he wouldn’t listen to me. (no matter what i said, he wouldn’t listen to me.)（引导让步状语从句）
he would believe whatever i said. （引导宾语从句）
whichever/no matter which you choose, the others will be offended.
take whichever you like. 这里句中的whichever在不能用no matter what替换。
最后，关于名词性从句的连接词要指出的是，根据句意，引导表语从句的还可以是as if/as though, because, as。如：
it looks/seems as if it’s going to rain
it was because i got up late.
things were not as they seemed to be.
但是，当reason作主句的主语时，引导表语从句只能用that，如：
the reason for my absence was that i was ill.
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