英语中的名词性从句引导词的选择

引导名词性从句的连接词可分为三类：
连接词：that,whether,if (不充当从句的任何成分）
连接代词：what,whatever,who,whoever,whom,whose,which.
连接副词：when,where,how,why
不可省略的连词：
1.介词后的连词
2.引导主语从句和同位语从句的连词不可省略.
That she was chosen made us very happy.
We heard the news that our team had won.
whether 与 if 均为 “是否” 的意思.但在下列情况下,whether 不能被 if 所取代：
1.whether 引导主语从句（包括从句在句首的位置）应当用whether,不用if,如：
Whether it is true remains a problem.
Whether he will come,I am not sure.
2.引导表语从句用whether,不用if,如：
The question is whether you should accept it.
3.whether可以引导从句作介词的宾语,而if 则不能,如：
I’m not interested in whether they’ll go or not.
It depends on whether we have got enough money.
4.whether可以直接跟动词不定式连用,而if 则不能,如：
I didn’t know whether to laugh or to cry.
She hasn’t decided whether to go or not.
5.引导同位语从句用whether,不用if,如：
The question whe大部分连接词引导的主语从句都可置于句末,用it充当形式主语.
It is not important who will go.
ther he’ll attend the meeting is essential.
1）由从属连词that引导的从句叫做名词性that-从句.that只起连接主句和从句的作用,在从句中不担任任何成分,本身也没有词义.
名词性that-从句在句中能充当主语、宾语、表语、同位语和形容词宾语,例如：
用 it 作形式主语的that- 从句有以下四种不同的搭配关系：
a.It + be +形容词+ that-从句
　It is necessary that…　　　 \x09\x09有必要……
　It is important that…　　　 \x09\x09重要的是……
　It is obvious that…　　　　 \x09\x09很明显……
b.It + be + -ed 分词+ that-从句
　It is believed that…　　　　　\x09\x09人们相信……
　It is known to all that…　　　\x09\x09从所周知……
　It has been decided that…　　 \x09\x09已决定……
c.It + be +名词+ that-从句
　It is common knowledge that…　 \x09……是常识
　It is a surprise that…　　 \x09\x09令人惊奇的是……
　It is a fact that…　　　　 \x09\x09事实是……
d.It +不及物动词+ that-分句
　It appears that…　　　　　　\x09\x09似乎……
　It happens that…　　　　　　\x09\x09碰巧……
　It occurred to me that…　　 \x09\x09我突然想起
1）由wh-词引导的名词从句叫做名词性wh-从句.wh-词包括who,whom,whose,whoever,what,whatever,which,whichever等连接代词和where,when,how,why等连接副词.wh-从句的语法功能除了和that-从句一样外,还可充当介词宾语、宾语补语和间接宾语等,例如：
主语：How the book will sell depends on its author.
直接宾语：In one's own home one can do what one likes.
间接宾语：The club will give whoever wins a prize.
表语：My question is who will take over the president.
宾语补足语：She will name him whatever she wants to.
同位语：I have no idea when he will return.
形容词宾语：I'm not sure why she refused their invitation.
介词宾语：That depends on where we shall go.
2）wh-从句作主语也常用先行词it做形式主语,而将wh-从句置于句末,例如：
　It is not yet decided who will do that job.
1）yes-no型疑问从句
从属连词if,whether引导的名词从句是由一般疑问句或选择疑问转化而来的,因此也分别被称为yes-no型疑问句从句和选择型疑问从句,其功能和wh-从句的功能相同,例如：
主语：Whether the plan is feasible remains to be proved.
宾语：Let us know whether/if you can finish the article before Friday.
表语：The point is whether we should lend him the money.
同位语：They are investigating the question whether the man is trustworthy.
形容词宾语：She's doubtful whether we shall be able to come.
介词宾语：I worry about whether he can pass the exams.
2）选择性疑问从句
选择性疑问从句由关联词if/whether…or或whethe…or not构成,如：
Please tell me whether / if they are Swedish or Danish.
I don't care whether you like the plan or not.