名词性从句
在简单句中，各种句子成分是由单词、词组、介词短语、非限定动词短语等构成的，如果一个句子由一个主句和一个（含一个以上）的从句构成，则成为复合句。在复合句中，从属句的种类取决于其在整个句子中所起的作用。根据这一点，从句可分为主语从句、宾语从句、表语从句、同位语从句、定语从句和状语从句。由于主语从句、宾语从句、表语从句和同位语从句相当于名词在句子中通常所起的作用，故被统称为名词性从句。由wh-疑问词引出的名词性从句不可用疑问句语序。
名词性从句常用下列关联词引导：
从属联词：that, whether , if
连接代词： who, whom, whose, which, what,. whose, whichever, whatever
连接副词：when, where, how, why
注: 在what引导的名词性从句中, 一般情况下what = all that
篱笆墙的影子歌词That was what/all that he had when he died.
I won’t tell you what/all that the president said at the meeting.
一、主语从句
主语从句在句子中起主语作用，一般位于主句谓语之前：
What you need now is experience.
When the meeting will be held remains a question.
Whether solar energy can be used on a large scale is a thing we have great interest in.
注: 从句做主语时，为了使句子更加平衡，通常用it作形式主语，而将真正的主语从句放在句末。
It is well known that the sea covers about three-fourths of the earth’s surface.
Is it certain that he will win the game?
农业致富好项目 It wasn’t without difficulty that he accomplished his goal.
if引导的主语从句不能位于句首。
Whether it costs too much relies on how badly you need it.
Whether she comes is still not known.
二、宾语从句
宾语从句就是用从句形式作宾语，一般位于主句中的及物动词、介词及某些形容词之后，有时用it作形式宾语，从句位置后移，连词that有时可省略。
They told me (that) math is very difficult.
He is confident (that) he will take his university degree in the coming year.
She asked if I could tell her why Paul left the university without a diploma.
We found it reasonable that we stop to have a rest.
I consider it necessary that they make the experiment one more time.
在think, believe suppose, expect等动词后的宾语从句若为否定意义时，可将其转换为对主句的否定，而在从句部分用肯定式，构成否定转移。
The doctor doesn’t think (that) the patient is capable of traveling in health condition.
= The doctor thinks (that) the patient isn’t capable of traveling in health condition
I don’t believe (that) you are right on this point.
=I believe (that) you are not right on this point.
If引导的宾语从句不能位于介词之后。也不能和or not连用。
It all depends on whether we can get their cooperation.
I wonder whether they need help or not.
三. 表语从句
表语从句就是用句子形式作的表语,位于系动词之后,连词that可省略;
whether是什么意思My problem is how we can reduce the cost in production.
The attraction between the sun and the earth is what keeps pulling the earth into a curved path.
The problem is (that) we don’t have enough accommodation for all the students.
愿有人陪你颠沛流离电视剧用as if (a though)作连词时,可用虚拟语气,也可用陈述语气.
It looks as if he has promised to come.
I feel as if it were the end of the world. (虚拟)
眩晕的原因If不能引导表语从句。
The question is whether they has got in contact with the headquarter.
妹妹美美网四、同位语从句
同位语从句是用句子形式表示的同位语,用于说明前面名词的内容，使之更加明确。同位语从句前的名词通常是一些需要进一步说明其内容的词，如：conclusion, fact, hope, idea, opinion, suggestion, news，reason, thought, belief, truth等。主要由that引导, 也可用what, where, how, when, why，whether等. 引导同位语从句的that 是连接词, 无词义, 属虚词,不能充当句子成分。If 不能引导同位语从句。
如:
I still have some hope that I will succeed.
I have no idea what time it is now.
The fact that he has spoken to the press about this caused us a lot of embarrassment.
Whether they will join us hasn’t been made clear.
状语从句
状语从句就是用句子形式表示的状语，用来说明主句中的谓语、状语、定语或整个句子。故而又称为副词性从句。带状语从句的复合句中，主句结构比较完整，没有它主句意思依旧成立。而在带有名词性从句的复合句中，该名词性从句是全句不可或缺的部分，否则句子意思就不完整。名词性从句在整个句子中的位置是较为固定的，一般不可更变。而状语从句可置于主句之前，也可置于主句之后，前置时常常用逗号点开，后置时一般不用逗号。
状语从句分为时间状语从句、地点状语从句、让步状语从句、原因状语从句、结果状语从句、目的状语从句、条件状语从句、方式状语从句、比较状语从句等九种。每种状语从句各自有一定的连接词语。
1. 时间状语从句
时间状语从句通常由下列连词引导：
When, while, whenever, since, after, as, once, until, as soon as, before, so(as)long as, till, the moment等, 例如:
Once you see her, you will never forget her.
While Betty was getting dressed, she kept looking at the clock.
When it rains, I usually go to work by bus.
注： 用 since (自……以来)引导时间状语从句时, 主句要用完成时态, since 从句则常用一般过去时态或现在完成时态, 若要说明某事发生以来至今的时间长度, 要用It is (It’s been) + 时间词语+since的句型结构,如:
I have come here several times since you left us.
It’s three since my daughter caught the flu.
注：连词when, while及as 都可表示“当……时”, 但用法有区别: 用When时, 从句和主句的动作可以同时发生,也可以先后发生; 用as , while 时则强调从句和主句的动作同时发生，从句的谓语用进行时态，表示“随着”, “正当”, “一边……一边”的意义，while 所引导的从句，谓语通常用延续性动词的一般时或进行时。
例如：
She dropped the letter when she sat down.
While my parents were watching TV, I was preparing for my lessons of the next day.
As the national economy is developing, our life has greatly improved.
2. 地点状语从句
地点状语从句用以下连接词引导：
where, wherever
When previewing a new lesson, make marks where you have questions.
Where there is a will, there is a way.
Wherever I am, I will be thinking of you.
3. 原因状语从句
引导原因状语从句通常由以下连词引导：
Because, since, as, now that, in that, for the reason that, seeing that等.
其中because “原因”语气最强, 表示有必然的因果关系，所引导的从句是整个句子的重心所在, 位置一般在主句后, 需要强调时, 也可前置。Since表示一种间接或附带的原因，可译为“既然”，as的语气更弱。
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