一、主语从句的概念与类型
主语从句在复合句中作主句的主语,引导主语从句的词有从属连词that, whether以及连接代词who, what, which, whoever, whatever, whichever和连接副词when, where, why, how等.例如:
That she was chosen made us very happy.她当选了使我们很高兴.
What caused the accident is a complete mystery.是什么导致了这次事故还完全是一个谜.
Whether she will come or not is still a question.她是否来还是一个问题.
Which team will win the match is still unknown.哪个队会赢得这场比赛还不知道.
Whoever comes to the party will receive a present.参加聚会的每一个人都将收到一份礼物.
When they will start has not been decided yet.他们何时出发还没决定.
注意:上述例句中的主语从句都是放在句首,但有时为了使句子结构平衡,避免"头重脚轻",常用it作形式主语,而把从句放在后面.例如:
It is a pity that she has made such a mistake.她犯了这样一个错误,真是遗憾.
It depends on the climate whether they are going shopping today.他们是否今天去购物还得看天气情况.
这样就构成了下面一些常用句型:
1)It is + adj. / n. +从句
It is a pity/shame that...遗憾的是……
It is possible that...很可能……
It is unlikely that...不可能……
2)It +不及物动词+从句
It seems/appears that...似乎……
It happened that...碰巧……
3)It + be +过去分词+从句
It is said that...据说……
It is known to all that...众所周知……
It is reported that...据报道……
It is believed that...据信……;人们相信……
It is suggested that...有人建议……
It must be pointed out that...必须指出……
It has been proved that...已证明…….例如:
It is believed that at least a score of buildings were damaged or destroyed.据信,至少有二十座楼房遭到破坏或彻底毁掉.
It is suggested that the meeting should be put off.有人建议会议延期召开.
It's reported that three people were killed in the accident and five were hurt badly.据报道,事故中三人丧生,五人重伤.
It is known to all that Taiwan is an inseparable part of China.众所周知,台湾是中国不可分割的一部分.

二、学习主语从句应该注意的几个问题
1)引导词that与what
what既有引导主语从句的作用,同时又在从句中作句子成分(主语、宾语、表语等).这时what相当于all that/everything that...,常译成"所……的(东西、事情、话等.)";而that只起连接作用,本身无实际意义,在从句中也不担当任何句子成分.但that引导的主语从句放在句首时,不能省略that.例如:
What is done can't be undone. (谚语)已成定局,无可挽回.
What we can't get seems better than what we have.我们得不到的东西似乎比已得到的要好.
That we should work out a plan to deal with the present serious situation is important.我们应该制定一项计划来应付目前严峻的形势,这一点非常重要.
That a new teacher will come to teach us geography is true.一位新老师将来教我们地理,这是真的.
2)引导词if和whether
if和whether引导动词后的宾语从句时一般可通用,但介词后的宾语从句、主语从句(放在句首时)、表语从句常由whether引导.例如:
Whether she will go home or not is unknown.她是否回家还不得而知.
Whether we shall raise ducks or geese remains to be decided.我们是养鸭还是养鹅仍没决定.
=It remains to be decided whether/if we shall raise ducks or geese.
但我们不能说If we shall raise ducks or geese remains to be decided.

三、语气
在It is necessary / natural / important / strange...+ that-clause这类句型里,that所引导的主语从句中谓语动词用"should +原形",表虚似语气.例如:
It is necessary that the problem should be settled at once.这个问题必须马上解决.
It's strange that he should have gone away without telling us.他竟然没向我们说一声就走了,真是奇怪.

3)其它引导词
连接代词who, which, whom, whose有各自的意义,起连接作用,并在从句中充当语、宾语、表语、定语等;连接副词when,where, why, how等有各自的意义,起连接作用,并在从句中充当状语.

四、以it作形式主语的主语从句与强调句型的区别
"It be + adj./n. + that-clause"与强调句型均有It be...that...之类的语言标志,但不同的是:①前者中的that从句是主语从句.若删掉其中的It be和that,则剩余部分不论结构还是语意都不能成为一个句子;但若将后者中的It be和that去掉,则剩余部分的结构和语意仍能构成一个完整的句子.②强调句型译为汉语时可加上"正是……"或"就是……"之类的字眼,而主语从句则不可以.请比较下面两个句子:
It is surprising that Mary should have won first place.令人惊奇的是玛莉竟然获得了第一名.
It is Mary that has won first place.正是玛莉得了第一名.
第一句话中的It是形式主语,that引导的是主语从句.句中的It is和that无法删除,一旦删除句子就不成立.
第二句是强调句型,其中的It is和that可以去掉,因为没有It is和that句子仍然很通顺.