定义：用作主语的从句叫主语从句，它是名词性从句之一。主语从句的时态：不受主句的时态影响和限制。引导主语从句的连接词有：
从属连词： that, whether；
连接代词： who (whoever), whom, whose, what (whatever)和which (whichever)；
连接副词： when, where, how和why。
连接词：1. 从属连词：that， whether
1）that引导主语从句只起引导作用，在主语从句中不充当任何成分，that本身无实际意义，但不能省略。
2）whether意为“是否”，放在句首句末皆可。由whether引导的名词从句做主语，表语，介词宾语，同位语时！不可用if代替。
price will go up is certain
b. whether they would support us was a problem. 他们是否会支持我们还是一个问题。
2. 连接代词who ，what ，which, whatever, whichever, whoever (who, whom, which, what可以和ever构成合成词，和what一样引导从句，ever起到强调作用。此类主语从句不能用形式主语it引导，它们在句子中担任成分，不能省略，语序为陈述语序，既有疑问含义，又起连接作用，同时在从句中充当各种成分。
which side will win is not clear. whoever breaks the law will be punished.
whatever he did is right. who killed the scientist remains a question.
1）what和that引导的主语从句的区别：that 只起引导作用，不充当任何成分，没有意思。what 不仅起引导作用，还要充当一定成分(主语、宾语或表语)，意思是：“什么”。
what we need is money. that he will come is certain.
2）whatever / whoever的用法（~ever=no matter ~-让步状语从句中）
whatever / whoever可以引导主语从句，并在句中作主语、宾语、表语等，不含疑问意义。whatever相当于anything that; whoever 相当于anyone who。例如：
whoever(anyone who) wants to enter into this school must take the exam.
whatever(anything that) she did was right.
3. 连接副词when，where， how，why等。既有疑问含义，又起连接作用，同时在从句中充当各种成分。
howthishappenedisnotcleartoanyone. when he will be back depends on the weather.
when we’ll start tomorrow will be told soon. where the test will be given is not yet decided.
4. 主语从句后置：为了避免主语冗长，句子头重脚轻，经常用 it 作形式主语，主语从句放在后面作真正的主语。
例： that we shall be late is certain. -it’s certain that we shall be late.
1）用it 作形式主语的结构。
+ be + 形容词 + that 从句 (obvious,true,surprising,good,wonderful,funny,possible,likely,certain,probable,etc.) 如：
itiscertainthatshewilldowellinherexam.
b. it + be + 名词词组 + that 从句 (anhonour,afact,apity,etc）如：
it is a pity/shame that...遗憾的是…… it is a surprise that…令人惊奇的是…
it is a fact that… …是事实 it is common knowledge that… …是常识。
it’sapitythatwecan’tgo.
+ be + 过去分词 + that 从句 (said,reported,thought,expected,decided,announced,etc。） 如：
+ 不及物动词或短语 + that 从句 ( seem，happen, doesn’tmatter ,turn out） 。如：
ithappenedthatiwasoutthatday. itdoesn’tmatterwhethershewillcomeornot.
2) 对于以连接代词(副词)引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句：
a. whether they would support us was a problem.
b. it was a problem whether they would support us.
3）主语从句中的否定前移。
当用it作形式主语，而将主语从句放在句尾时，主语从句中的否定词常要前移至主句中。例如：
it doesn’t seem that they are from the same university.
it doesn’t appear that we’ll h**e a sunny day tomorrow.
5. 注意：
1）主语从句在句首时，必须由连接词引导，不能省略这些连接词。
误：they should like each other is natural. 正：
that they should like each other is natural.
2)在 it + be + suggested/advised/ordered/requested/ insisted/required 中， that从句的谓语也应该用“should + 动词原形”的形式， should有时可以省略。主语从句中用陈述语序，后面的谓语动词用第三人称单数。 it is suggested that we (should) do the experiment a second time.
3) .用于 it is important / natural / necessary / impossible that...句型中， 主语从句应使用虚拟语气， 用“should + 动词原形”的形式， should有时可以省略。
it is important and necessary that we (should) keep the balance of nature.
4）“what”引导主语从句时，谓语动词：常与其后的名词作表语一致，且根据句子的语境而定。
eg：what you left are only several old books. what you said is of great importance.
5)在it is funny, strange, surprising, a pity, a shame, no wonder, etc. +that从句中，有时谓语部分用should + 动词原形，来表达说话者的感**彩，此时should也可省略。
6)语气。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。
it is surprising that mary should h**e 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。【强调】
6..主语从句的规律：
例 will he come is not known. 正：when he will come is not known.
规律。一、主语从句一律用陈述语序，即主语在前，谓语在后。
例2. he will not come to the meeting this evening is true.
正：that he will not come to the meeting this evening is true.
规律。二、连词that 在从句中无实际意义，但不能省略。
例3. if the meeting will be put off has not been decided yet.
正： whether the meeting will be put off has not been decided yet.
规律三、whether 可以引导主语从句，放在句首，但if不能。
例4. that whether he will help others is a fact.
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