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在复合句中充当主语成分的句子叫做主语从句。宾语从句，是名词性从句的一种。在主从复合句中充当宾语，位于及物动词、介词或复合谓语之后的从句称为宾语从句。宾语从句分为三类：动词的宾语从句，介词的宾语从句和形容词的宾语从句。
1．从句为陈述句，常选择引导词that或将that省略，直接与主句相连。
2．从句为一般疑问句，常选择引导词if或whether。在whether…or not结构中不能用if替换。
3．从句为特殊疑问句，常选择what,when,where,which,who,how等疑问代词、疑问副词作引导词。 注意：当who为主语时，句式为：who+谓语+其他。从句的引导词有很多，如：say, think, wish , hope, see, believe, agree, expect, hear , feel等。
由that引导的陈述句性的宾语从句，在很多动词如say, think, wish ,hope, see, believe, agree, expect, hear ,feel等动词后。
连词that只起连接作用，在从句中不做句子的成分，也无词汇意义，在口语中常被省略，但在大多数情况下还是以不省为好，特别是在笔语中。
例：I told him that he was wrong.
在think,believe,suppose,expect等动词引起的宾语从句中，有时谓语尽管是否定意义，却不用否定形式，而将think等动词变为否定形式。
例：I don’t think you are right. (我认为你做的不对)
在许多带有复合宾语的句子中，that引导的宾语从句经常移到句子的后面，而用it做形式宾语。
例：We think it wrong that he told a lie to everyone (我认为他向每一个人撒谎是错误的)
B,由连词if、whether引导的表示“是否…”的宾语从句。Whether,if在从句中不做句子的成分，一般情况下，whether和if可以替换。
例：I don’t know if/whether he will come tomorrow.
The teacher asked if/whether we had
finished the experiment.
宾语从句的简化情况
1、当主句谓语动词是hope，decide，wish，choose，agree，promise等，且宾语从句的主语与主句主语一致时，宾语从句可简化为不定式结构。例如：
Li Ming hopes he will be back very soon.→Li Ming
hopes to be back very soon.
We decided that we would help him.→We decided
to help him.
2、当主句谓语动词是know，learn，remember，forget，tell等动词，且主句主语与从句主语一致时，宾语从句可简化为“疑问词+不定式”结构。例如：
She has forgotten how she can open the
window.→She has forgotten how to open the window.
3、当主句谓语动词是tell，ask，show，teach等动词，且后带双宾语，从句主语和间接宾语一致时，宾语从句可简化为“疑问词+不定式”结构。例如：
Could you tell me how I can get to the
station？
→Could you tell me how to get to the station？
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4、当主句的谓语动词是order（命令），require（需要）等时，如果主句和从句的主语不一致，宾语从句可简化为“名词（代词）+不定式”结构。例如：
The headmaster ordered that we should start
at once.→The headmaster ordered us to start at once.
5、某些动词后的宾语从句，可以用介词加动名词（短语）等其他形式简化。例如：
He insisted that he should go with us.→He insisted
on going with us.
The poor boy doesn’t know when and where he
was born.→The poor boy doesn’t know the time and the place of his birth.
宾语从句引导词不能省略情况
（1）当句中的动词后接多于两个由that引导的宾语从句时，第一个that可省，但后面的that不可省。He said (that) you were too young to understand the matter and that
he was asked not to tell you.
（2）当主句的谓语动词与that宾语从句之间有插入语时，that一般不可省。Just then I noticed, for the first time, that our master was wearing
his fine green coat and his black silk cap.
（3）当that从句是双宾语中的直接宾语时，that不可省。I can’t tell him that his mother died.
注意事项：许多带复合宾语的句子，that引导的宾语从句经常移到句子后部，而用it作形式宾语。I find it necessary that we should do the homework on time.
需要额外注意的是，由whether和,If引导的宾语从句
由whether(if)引导的宾语从句，实际上是一般疑问句演变而来的。意思是“是否”。宾语从句要用陈述句语序。一般说来，在宾语从句中whether与if可以互换使用，但在特殊情况下if与whether是不能互换的。
