同位语从句的概念：

在复合句中充当同位语的名词性从句称为同位语从句。同位语从句是名词性从句(主语从句、表语从句、宾语从句、同位语从句)中的主要从句之一，从句作同位语表示与之同位的名词(短语)的实际内容，它的作用相当于名词，对前面的名词(短语)加以补充说明或进一步解释，相当于一个表语从句，它们之间的关系是同位关系，即主表关系。

同位语从句的用法：

1、同位语从句的引导：
词引导同位语从句的词语通常有连词that，whether，连接代词和连接副词等：
如：We heard the news that our team had won. 我们听到消息说我们队赢了。   
        They were worried over the fact that you were sick. 他们为你生病发愁。 
        I have no idea when he will come back. 我不知道他什么时候回来。
        There is some doubt whether he will come. 他是否会来还不一定。 
        The question whether it is right or wrong depends on the result. 这个是对还是错要看结果。
注：whether可引导同位语从句，但if不能引导同位语从句。

2、同位语从句与定语从句的区别：
（1）意义的不同：
 同位语从句是用于说明所修饰名词的具体内容的，它与被修饰词语通常可以划等号；而定语从句是限制所修饰名词的，它的作用是将所修饰的名词与其他类似的东西区别开来：
如：We are glad at the news that he will come. 听到他要来这个消息我们很高兴。
(news的内容就是that he will come，故that引导的是同位语从句)
        We are glad at the news that he told us. 听到他告诉我们的这个消息我们很高兴。
(that从句是限制thenews的内容的，即我们高兴只是因为他告诉的这个news而不是其他的news，故that从句为定语从句)
（2）引导词的不同：
          what, how, if, whatever等可引导名词性从句，但不引导定语从句。
（3）引导词的功能上的不同：
          that引导同位语从句时，它不充当句子成分，而引导定语从句时，它作为关系代词，要么充当定语从句的主语，要么充当定语从句的宾语。
如上例that he told us中的that就充当told的宾语。
（4）被修饰词语的区别：
          同位语从句所修饰的名词比较有限，通常有hope, wish, idea, news, fact, promise, opinion, suggestion, truth等，而定语从句所修饰的名词则非常广泛。
另外，when和where引导定语从句时，通常只修饰表示时间和地点的名词，而它们引导同位语从句时却不一定；又如why引导定语从句，它通常只修饰名词thereason，而它引导同位语从句时则不一定：
如：I have no idea when they will come. 我不知道他们什么时候来。(同位语从句)
        I'll never forget the days when I lived there. 我永远不会忘记我住在那儿的日子。(定语从句)
        We don't understand the problem why this is the best choice. 我们不明白这个问题，为什么这是最好的选择。(同位语从句)
        There a son why he didn't come to the meeting is that he is ill. 他未能来开会，原因是他生病了。(定语从句)

同位语从句用法解析：

一、理解同位语从句的含义，把握同位语从句的实质：　　
在主从复合句中作同位语的从句称为同位语从句。同位语从句一般用that, whether, what, which, who, when, where, why, how等词引导，常放在fact, news, idea, truth, hope, problem, information, wish, promise, answer, evidence, report, explanation, suggestion, conclusion, word, possibility等抽象名词后面，说明该名词的具体内容。换言之，同位语从句和所修饰的名词在内容上为同一关系，对其内容作进一步说明。
例：The news that they had won the game soon spread over the whole school. 他们比赛获胜的消息很快传遍了整个学校。
析：they had won the game说明the news的全部内容，因此该句为同位语从句。

二、正确运用同位语从句的引导词，准确把握同位语从句：
1、如同位语从句意义完整，应用that引导同位语从句。（即that不充当任何成分，只起连接作用，不可省略）　
例：The general gave the order that the soldiers should cross the river at once. 将军下达了战士们立即过河的命令。　
析：the soldiers should cross the river at once是the order的全部内容，且意义完整，因此应用that引导同位语从句。
2、如同位语从句意义不完整，需增加"是否"的含义，应用whether引导同位语从句。（if不能引导同位语从句）
例：We'll discuss the problem whether the sports meeting will be held on time. 我们将讨论运动会是否会如期举行的问题。
析：the sports meeting will be held on time 意义不完整，应加“是否”的含义才能表达the problem的全部内容，因此应用whether引导同位语从句。
3、如同位语从句意义不完整，需增加"什么时候"、"什么地点"、"什么方式"等含义，应用when, where, how等词引导同位语从句。　
例1：I have no idea when he will be back. 　
析：he will be back 意义不完整，应加"什么时候"的含义才能表达idea的全部内容，因此应用when引导同位语从句。　
例2：I have no impression how he went home, perhaps by bike.
析：he went home 意义不完整，应加"如何"的含义才能表达impression的全部内容，因此应用how引导同位语从句。
4、当主句的谓语较短，而同位语从句较长时，同位语从句常后置。
如：The thought came to him that may be the enemy had fled the city.

it的概念：

it可用作人称代词、指示代词、先行词及引导词等。

it 的用法：

1、it可指天气、温度、时间、距离等 。
如：It is cold today, isn't it?
2、用作人称代词，代替前文提到过的事物。
如：The dog is not  acold-blooded animals.  It doesn't need to hibernate.
3、为避免重复，it可用来代替前面说过的短语或句子。
如：I tried to persuade my father to give up smoking, but found it impossible. (it=to persuade my father to give up smoking)
4、代替指示代词this，that。
如：—What's this?
        —It's an album.
        —Whose new bike is that?
        —It's Mary's.
注：it与one，that的区别：
        it=the（this, that）+名词，特指并且代替前面所提到的某特定事物。
如：He's bought a new car, so he drives it everywhere to show ito ff.
        one=a+名词，one指前面提到的同类事物中的不同的另一个。
如：He needs a computer, but he can't afford one.
        that=the+名词，that指代的名词与前面的名词属于同一类，但不属同一个。
如：The population of China is larger than that of Japan.  that指代population，但其后有一个of短语作定语，以区别于the population of China。
注：it与that的异同：
        it指同一事物，that指同类但并不是同一事物。
如：I like the climate of Kunming more than that of Beijing.
       The climate of Kunming is mild, and I like it.
5、It/This/That+be+the first(second, third...) time+that-clause 这个句型表示截止到说话时为止的某人的一种经历，关键是注意time前有序数词，主句是一般现在时is时，从句要用现在完成时；如果主句用一般过去时was时，则从句须相应地用过去完成时。
如：This is the first time(that) these Europeans have visited the Great Wall.
        It was the fifth time(that) I had paid a friendly visit to America.
6、在一些相对固定的词组中，没有特殊含义，经常不翻译。
如：He's never really made it as an actor. 作为演员，他从未获得过真正的成功。
        It is my turn. 轮到我了。

强调句中的it：

可以用来改变句子结构，使句子的某一成分得到强调：
1）强调句的基本句型it's/was+被强调成分+that/who+其他成分
原句：I told Jim the news in our office yesterday.
强调主语：It was I that/who told Jim the news in our office yesterday.
强调宾语：It was Jim that I told the news in our office yesterday.
            或：It was the news that I told Jim in our office yesterday.
强调地点状语：It was in our office that I told Jim the news yesterday.
强调时间状语：It was yesterday that I told Jim the news in our office.
2）强调句的一般疑问句型Is/Was+it+所强调部分+that/who...?
如：Was it you that told Jim the news in your office yesterday?
3）强调句的特殊疑问句型疑问词+is/was+it+that/who...?
如：Who was it that told Jim the news in your office yesterday?
【注】强调句与主语从句虽然在形式上很相似，即都含有it is(was)...that。但，区别在于：强调句去掉it is(was)…that之后，句子结构仍然完整，而主语从句却不能这样。
如：(It is)our hope(that) the two sides will work towards peace.
解析：去掉It is…that之后，句子是不成立的。由此得出该句不是强调句，而是一个简单的主语从句，it是形式主语，从句是真正的主语。

“it”的用法：

1、it 作形式主语：
it 在句中可作形式主语，而真正作主语的主语从句需要放在句子的末尾。主语从句后置常用以下几种结构：
1）It is/was+adj.+subject-clause可用于此句型的形容词有：
clear, certain, funny, good, impossible, likely, natural, obvious, possible, probable, strange, surprising, true, unusual, wonderful等。
如：It is obvious that going for sports will do a lot of good to your health.
2）It+be+adj./n.(forsb./ofsb.)+todosth. 该句型中的形容词通常表示事物的特点或特征的，如：
difficult, hard, easy, impossible, necessary, important等，此时用for；或表示人的性格特征或特点的，如:
nice, good, bad, kind, silly, foolish, wise, clever, careless, rude, brave, cruel, careful, grateful等，这时要用of 。
3）It is/was+名词词组+subject-clause可用于该结构的名词词组有：
a pity/duty, a good thing, no surprise, good news, an honor, a fact, a mystery, a shame, manners等。
 如：It's a pity that I didn't attend the party.
4）It is/was+V-ed+subject-clause可用于该结构的动词的过去分词有：
said, reported, thought, supposed, believed, hoped, expected, known, decided, announced, arranged等。
如：It is said that something had been done to end the pollution.
注：本句还可改写为：Something is said to have been done to end the pollution.
5）It+vi.+subject-clause可用于该结构的动词有：appear, seem, happen, occur等。
 如：It appeared to scientists that the stars had moved.
6）It doesn't matter(makes no difference,etc.) +连接代词或连接副词引起的从句作宾语。
如：It doesn't matter whether he'll join the army or not.
        It makes no difference where we have the conference.
7）一些固定句型：
It takes sb. some time to do sth.
如：It will take you two days to get there on foot. 
It costs sb. some money to do sth.
如：It costs 1,000 dollars to fly to America.
It is/was no use(useless) doing（做什么是没有用处的）
如：It's no use arguing with him.
It is/was no good doing（做什么是没有好处的）
8）以下句型结构中需要用虚拟语气
① It is/was important(necessary, strange) that...;
     It is/was ordered(required, suggested, proposed) that...;
     It is/was a pity(a shame) that...表示遗憾等感情的句子中，主语从句要用“should+动词原形”，should可以省略。
如：It's necessary that he(should) be operated on at once.
② It is (high)time that...结构中用should+动词原形（should不能省略）或动词过去式。
如：It is high time that you should make(made) full use of your time to go over your lessons.

2、it作形式宾语：
1）动词consider（feel, find, think等）+it+形容词（名词）+不定式（动词-ing形式，从句）。
 如：She thinks it no use telling me.
2）主语+appreciate（enjoy，like，love，hate）+it+if（when）...结构
如：We would appreciate it if you could come to help us.
3）dependon, relyon, see to（负责/设法做到）, takeforgranted（习以为常）等短语后跟that从句时，要以it作形式宾语。
如：We're depending on it that he will finish the job by Friday.

“it ”引起的几个易混淆的时间句型：

1）It be+时间+since-clause 这个句型表示从since从句谓语动作发生以后到现在或过去所经过的一段时间，意为“自从…以来已多久了”，主句多用一般现在时，从句用一般过去时，如果表示过去的情况，主句一般用过去时，从句用过去完成时，或主句用过去完成时，从句用一般过去时。
注：since引导时间状语从句时，从句若使用终止性动词，则表示该时间是主句时间段的终点（时间从现在算起）；若从句使用延续性动词，则表示该动作状态的结束（时间从过去算起）。
如：It's five years since they got married. 他们结婚已经5年了。 
        It's five years since they were married. 他们离婚已经5年了。
        It's ten years since his father was a worker. 他父亲不当工人已经10年了。
        I haven't seen him since we were boys together. 我们长大以后再没有见过面。 
2）It be+时间+before-clause 这个句型中的时间一般为表示一段时间的词语（如：long years, months, weeks, days, hours, minutes），主句中的谓语动词用肯定式，意为“过多长时间才…”。主句的谓语动词是否定式时，意为“没过多长时间就…”。主句的时态可用过去时was或将来时willbe；用was时，before从句的动词用一般过去时；用willbe时，before从句常用一般现在时。 如：It was not long before she learned those poems by heart. 她没过多久就背会了那些诗。
        It was long before the police arrived. 过了很久警察才来。
        It will be hours before he makes a decision. 要过好几个小时他才会作出决定。
        It will not be hours before we meet again. 要不了几个小时我们还会再见面的
3）It be+时间+when-clause 这个句型中，it指时间，而且表示时间的词语前没有介词（时间一般是具体时间）。主句和从句中的谓语动词在时态上是一致的，主句是willbe，when从句用一般现在时代替将来时。
如：It was already 8 o'clock when we got home.
        It will be late afternoon when they get there. 
4）It be+时间+that-clause 这个句型是个强调句型。
如：It was at 5o'clock that he practiced playing the violin in the morning. (原句是：He practiced playing the violin at 5o'clock in the morning.)
比较：It was 5o'clock when he started in the morning.（5o'clock前没有介词，这个是定语从句）
5）It be+time+that-clause 这个句型属虚拟语气结构，不管主句中用的是is或was，that从句都须用动词的过去式或should+动词原形（但不及物动词通常用过去式），在time之前有时可以加上high 或about 以加强语气。
如：It is high time(that) he wrote a letter to his girl friend.
        It is time(that) we made people's life a little easier.= It is time that we should make people's life a little easier.