That he will come to the discussion is certain.
That the moon moves round the earth is well known to all of us.
When the meeting is to be held has not yet been decide.
主语从句放在句首，句子常常显得比较笨重，因此通常可以把it放句首，作形式主语，将主语从句放在后面。
It is certain that he will come to the discussion.
It is well known to all of us that the moon moves round the earth.
在主句中作谓语动词、介词及非谓语动词的宾语的主谓结构称之为宾语从句。
引导宾语从句时，从属连词that在口语中和非正式文本中常省略。
We know that a parrot can’t really speak.
He wants to tell us what he thinks.
Knowing that it was going to rain, they decided to work inside the house.
知道要下雨了，于是他们决定在室内工作。(现在分词knowing的宾语从句)
I want to know whether/if you still work in the factory.
我想知道你是否还在工厂工作。（不定式to know的宾语从句）
The club will give whoever wins a prize.
Up to now we can’t say whether his theory can stand the test.
有些形容词可以用that引导宾语从句，表示说话人对某一事物的态度，带有感情色彩。常见的这类形容词有：
afraid, amazed, astonished
aware, certain, confident
delighted, glad, grateful
I am glad that you have come.
I am afraid that you are wrong on this point.
He is quite confident that he will pass the examination.
One idea is that fish is the best brain food.
The problem is where we can hold our meeting.
My question is how information is stored in the long-term memory.
That is what he really wants.
It seems that everything goes smoothly.
同位语从句用于对前面出现的名词作进一步说明，一般用连词that引导，由于先行名词的意义不同，也可用whether, who, when, where, what, why, how等引导。
evidence, suggestion, order
problem, report, decision.
I had no idea how many books I could borrow at a time.
The news came that their team had won the championship.
1. that 引导主语从句时，that 没有意义，但不能省略。
It worried her a bit that her hair was turning gray.
that引导主语从句，it作形式主语，that不能省略。
That she is a rich woman is known to us all.
2.从句作主语时，多数情况下由 it 作形式主语，而把主语从句放在后面，尤其是谓语部分（包括宾语）较短的情况下。
例句：
It wasn’t very clear what she meant.
It is important that he should come on time.
It is true that that man on the left is a well-known writer here.
这里，第一个that 引导主语从句，是单纯的连词，无词义，不作句子成分。第二个that 是指示代词，作句子成分，是man 的定语。
3.whether既可以引导主语从句也可以引导宾语从句，但if不能引导主语从句。
whether 后面可以加or not, 而if 不能与or not 连用。作介词宾语时不用if.
Whether I knew John doesn’t matter.
= It doesn’t matter whether I knew John.
Whether or not she’ll come isn’t clear.
= Whether she’ll come or not isn’t clear.
= It isn’t clear whether ….
seem,appear,happen,look,matter,
It appeared that he had a happy childhood.
It made us very happy that she was saved.
她能化险为夷令我们大家很高兴。
true,strange,necessary,important,
certain,wonderful,possible,likely,probable
It is certain that free medical care
will be given to mostpeople in our city.
It is obvious that he has been treated.
很明显他已经被款待了。
fact,surprise,pity,honor,common knowledge
It is common knowledge that the moon cannot give out light
It is a surprise that you gave such an answer to the question
said,reported,believed,known,
decided,thought,expected, announced
It is said that Jack has been to Europe.
It was reported that a new park will be built.
据报道将要建造一个新的公园。
需要注意的是，作为形式主语的it并无实际意义，只是为了满足语法上的需要，避免句子头重脚轻，它代替的是句子的逻辑主语，而真正主语由从句或不定式充当。
如果定语从句的时态与主句的时态具有同时性（包括谓语有情态动词）或之后性，则可以用“关系代词或关系副词＋不定式”来简化，或直接用“不定式”来修饰先行词。
We moved to the country so that the children would have a garden in which they could play in.
We moved to the country so that the children would have a garden in which to play.
(1) 省略“关系副词＋主语＋be动词”或“关系代词＋be动词”
We have never forgotten the days (when we were) at college.
I don’t know the person (who is) in your office.
He is not a person who likes to haggle over every ounce.
He is a person above personal interests.
（1）“现在分词短语”与前面的名词一定是主谓关系；
例如：谓语是一般现在时或将来时，现在分词所隐含的是现在时、现在正在继续时；如果谓语动词为一般过去时或将来时，现在分词所隐含的时态为过去时、过去正在进行时；
（3）being＋-ed表示“正在进行时的被动语态”。如果信息的中心在什么时候可以用现在分词呢？
当被修饰的名词为“不定代词”、“泛指意义的名词”或“专有名词”，可用现在分句短语作后置定语由于主语是泛指，句子的意思往往表达的是一条“道理”或一件“事实”，所以它们的隐含时为“一般现在时”，此时就可以用分词短语作后置定语。为了看清分词所隐含的时态，配上定语。
为了大家看得明白，配上定语从句，把分词所替代的时间展示出来
Anyone touching that wire will get an electric stock.
任何人碰到那根电线就会触电。
=Anyone who touches that wire will get an electric stock.
注：虽然被修饰的词是泛指意义的词，但是如果从句里的时态不是同步，则不可以用“现在分词”去作后置定语，只能用“定语从句”。
Do you know anybody who has lost a dog?
你知道有谁丢了一只猫？
定语从句所修饰的先行词可以是名词或代词，也可以是一个句子。定语从句通常位于先行词之后，由关系代词或关系副词引导。
限制性定语从句修饰先行词，对先行词起修饰作用，紧接先行词之后，无逗号，若省去，原句意思不完整。
引导定语从句的关系代词有who,whom,whose,which,that等。
who,whom,whose用于指人，whose有时也可指物，相当于of which；which用于指物；that既可指人也可指物，但只用于限制性定语从句中。
例句：
The computers and cables which make up the Internet are owned by people and organizations.
Those who live alone or who are sick may have trouble in getting close to other people.
The girl whose parents died in an accident is living with her
1. 当先行词是all,anything,everything,something,nothing等不定代词。
That is all that I've heard from him.
He's the first person that I'm going to interview this afternoon.
在从句中作宾语的关系代词常可省略。关系代词紧跟介词，作介词宾语时不可用that，只可用which或whom引导从句，并且不可省略。
This is one of those things with which we have to put up.
This is one of those things (whichhat) we have to put up with.
3. 引导定语从句的关系副词有when,where,why等。
关系副词在从句中作状语，意义上相当于一个“介词+which”的结构。
No one knows the reason why(=for which) he was so angry that day
非限制性定语从句既可修饰先行词，也可修饰整个主句，起补充说明作用，与主句之间有逗号隔开，若省去，原句意思不受影响。不可用that引导非限制性定语从句。关系词不可省略。
Every object has a gravitational pull,which is rather like magnetism.
“介词+whichwhomwhose”引导的定语从句
“介词+whichwhomwhose”可引导限制性定语从句，也可引导非限制性定语从句，该结构中介词的选择取决于从句谓语动词的固定搭配，或先行词的习惯搭配。
例句：
This is the computer on which he spent all his savings.
It is written by a person with whom we are all familiar.
as引导的定语从句主要用于“such...as”及“the same...as”的结构中，代替先行词是人或物的名词。as引导非限制性定语从句时，代替整个主句，从句可位于主句之前、之后或中间。
These are not such problems as can be easily solved.
As is mentioned above,no single company or group can control what happens on the Internet.(as代替主语）
关系副词可代替的先行词是时间、地点或理由的名词，在从句中作状语。
1. when, where, why 关系副词when, where, why的含义相当于"介词+ which"结构，因此常常和"介词+ which"结构交替使用。
There are occasions when (on which) one must yield.
Beijing is the place where (in which) I was born.
Is this the reason why (for which) he refused our offer?
that可以用于表示时间、地点、方式、理由的名词后取代when, where, why和"介词+ which"引导的定语从句，在口语中that常被省略。
His father died the year (that / when / in which) he was born.
He is unlikely to find the place (that / where / in which) he lived forty years ago.
关系代词所代替的先行词是人或物的名词或代词，并在句中充当主语、宾语、定语等成分。关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。
1. who, whom, that
这些词代替的先行词是人的名词或代词，在从句中所起作用如下：
Is he the man who/that wants to see you?
他就是想见你的人吗？（who/that在从句中作主语）
He is the man whom/ that I saw yesterday.
他就是我昨天见的那个人。（whom/that在从句中作宾语）
2. Whose 用来指人或物，(只用作定语, 若指物，它还可以同of which互换）
例句：
They rushed over to help the man whose car had broken down.
Please pass me the book whose (of which) cover is green.
它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等。
引导地点状语从句的连词是where,wherever.
1.引导原因状语从句的从属连词有：because,as,since,now(that),seeing that,considering that,in that等。
so...that, such...that , so that,that,so等。
Mickey Mouse is so attractive that the children are reluctant to leave.
so that,in order that,for fear that,lest等，
从句常使用may,might,can,could,would等情态动词。
We got up early this morning so that we could catch the first bus to the railway station.
if,unless, as(so) long as,
on condition that,in case,
provided(providing) that,
As long as you have the right equipment,you can use a telephone line to transmit computer data.
(whenever,wherever,however) 等
though,even if等引导状语从句可转换成含有as的部分倒装结构，具有强调意义。
其结构为“形容词（副词、动词、名词）+as+主语+谓语”。
No matter what you may say,I would not change my mind.
We have learnt quite a lot about it since we came here.
when、as、while、as soon as、before、after、since、till、until
the minute、the moment、the second、every time、immediately、the day、 directly、no sooner…than(一……就……)、hardly…when(刚一……就……)、scarcely…when(几乎没有…的时候)
特殊引导词：wherever、anywhere、everywhere
常用引导词：because、since、as、for
特殊引导词：seeing that、now that、in that、considering that、given that
特殊引导词：such that、to the degree that、to the extent that、to such a degree that
特殊引导词：to the degree that、to the extent that、to such a degree that、in order that
if、unless、whether(whether…or not)
as/so long as、only if、providing/provide that、supposing that、in case that、on condition that
though、although、even if、even though
as(用在让步状语从句中必须要倒装)、while(一般用在句首)、no matter…、in spite of the fact that、whatever、whoever、wherever、whenever、however、whichever
常用引导词：as(同级比较)、than(不同程度的比较)
the more…the more…、just as…、so…、A to B is what/as C is to D、no…more than、not so much A as B
that与what都可引导主语从句，that在从句中不作句子成分，但不能省略。what 在从句中可作主语、宾语、表语等，不能省略。
that与what都可引导宾语从句，that在从句中不作句子成分，一般情况下可以省略，但当有两个以上的宾语从句时，只能省掉第一个that。what 在从句中可作主语、宾语、表语等，不能省略。
that与what都可引导表语从句，that在从句中不作句子成分，且不能省略；what 在从句中可作主语、宾语、表语等，不能省略。
that与what都可引导同位语从句修饰表示抽象概念的名词，对所修饰的名词的具体内容进行详细阐述。这类名词有：
fact, idea, news, hope, belief, promise, thought, suggestion, proposal等。that在从句中不作句子成分，但不能省略。what 在从句中可作主语、宾语、表语等，不能省略。
that可引导定语从句且在定语从句中作主语或宾语，that既可指人也可指物。that在从句中作宾语时可省略。
如不能说：
He did all what he could to save the patient．
All what he needs is more time．
应将句中的what改为that，或去掉what，还可以将以上两句中的all去掉。
so．．．that, such ..．that, so that结构引导状语从句。what不能引导状语从句，但whatever, no matter what可引导让步状语从句。
例句：
Put the little plants in the shade so that the sun won’t burn them.
Don’t believe him no matter what he says．
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