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主语从句：即在复杂句中充当主语成分的句子例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶, 此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此"That he finished writing the composition in such a short time" 便是该复合句的主语从句。
主语从句使用陈述语序，其通常结构为“引导词+ 主语+ 谓语+ 其他成分”；但当连接代词同时充当从句主语时，其结构为“连接代词+ 谓语+ 其他成分”。
时态：主语从句的时态不受主句时态的影响和限制引导主语从句的连词主要有:从属连词:that、whether （只起连接作用，不充当句子成分）连接代词:who、whoever、whom、whose、what 、whatever 、which 、whichever （既有连接作用，又做从句的主语、宾语、表语或定语）连接副词:when、where 、how 、why whenever、wherever（既有连接作用，又做从句的状语）例句：1. that引导例句：That you are coming to London is the best news I have heard this long time. 你将来伦敦是好久以来我听到的最好消息。
2. whether引导例句：Whether they would support us was a problem. 他们是否会支持我们还是一个问题。
3. 连接代词引导例句：Whoever is top from wins the game when two matched players meet. 两强相争勇者胜。
语法——Noun clauses as the subject主语从句如果一个句子在复合句中充当一个主语, 那么这个句子就是主语从句。
①That he finished writing the composition in such a short time surprised us all.②What we need are good doctors.③Whom we must study for is a question of great importance.1.主语从句的连接词1).连接词: that, whether①.That he will win is certain.②.Whether he’ll join us in the discussion is of great importance.2).连接代词: who, what, which, whom , whose, whoever, whatever, whichever①.It is clear who is afraid of having a change in land ownership.②.What India needs is a law to make land ownership fairer.3).连接副词: when, where, why, how, how many, how much , how long, how soon, how often①When we’ll start tomorrow will be told soon.②Where we can buy oxen is something we need to find out.③Why he came here is not known.④How we can protect the grain from damp needs to be discussed.【连接词】1. that——That t he earth moves around the sun is well known.2. whether——Whether he will come or not is not known yet.3. who ——Who will be sent there has not been decided.4. what——What you need is more practice.5. which——Which one is the best is not clear.6. when, where ——Where he has gone is a question.7. why, how——Why there is gravity is hard to understand.How he can earn a living in Britain remains a big problem.8. whoever、whatever、whichever可引导主语从句。
英语语法知识讲解之名词性从句:主语从句主语从句有三类：1．由what 等连接代词引导的主语从句由what 引导的主语从句表示“…….所…….的东西”，在结构上等于一个名词加一个定语从句，what 在主语从句中要充当句子成分。
如 What we need is more time.我们所需要的是更多的时间。
what 在主语从句中作 need 的宾语，此句可改写成定语从句 :The thing which we need is more time.What is dif ficult is to do good all one’s life and never do anything bad.难的是一辈子做好事，不做坏事。
（what 在从句中作主语）由 whatever, whoever 引导的主语从句也属于这一类。
如 Whatever was said here must be kept secret.这里说的话都必须保密。
Whoever fails to see this will make a mistake. 谁看不到这一点就会犯错误。
2.由连词that 引导的主语从句在 that 引导的主语从句中，that 不但当句子部分。
如 That she will come is certain. 她来是肯定的。
That he was chosen made us very happy.他被选中了使我们都很高兴。
在更多情况下，这类主语从句都放在句子的后部，而用 it 作形式主语。
如 It is certain that you will be able to make greater progress.你一定能取得更大的进步。
It’s obvious that he is wrong.很明显，他错了。
It’s a pity that you missed such a fine talk.这样好的报告你没听到真遗憾。
