注意：that在从句中无词义，只起连接作用；连接代词和连接副词在句 中既保留自己的疑问含义、又起连接作用，在从句中充当成分。
4.当主语过长时，为了符合英语语言的习惯和避免出现头重脚轻的现象 常用it 代替主语从句作形式主语放于句首，而把主语从句置于句末；
It is known to all that light travels in straight lines.
A.so that B.that C.what D.in which
解析：what做连接词的时候永远等于“the thing(s) that”，it作形式 主语是只能代名词性结构，代不了“名词+定语从句”的结构。
单个的主语从句作主语时，谓语动词用单数形式；如果是两个或两个以 上的主语从句作主语，谓语动词则用复数形式。
(2) It is +形容词+从句 It is +形容词+从句 It is necessary that … 有必要…… It is clear that … 很清楚…… It is likely that … 很可能…… It is important that … 重要的是……
注意:在主语从句 中用来表示惊奇、 不相信、惋惜、理 应如此等语气时， 谓语动词要用虚拟 语气 “(should) +do”，如 strange， natural…
2.由连接代词who,that引导;
What we lack is experience. Who will go to the energy conference is not important.
3.由连接副词when,how,where,why引导;
How he manages to finish the job is of interest to us. Why he failed the english exam wasn't clear.
Where and when he was born has not been found.
When the person was murdered and why he was murdered are still unknown.
Predicative Clause(表语从句)
定义：指在复合句中做主句表语的从句。表语从句和主语指同一内容， 它对主语进行解释、说明，使主句的内容具体化。
It was a great achievement to complete a 24-story building in 10 months.
固定用法和译法: (1) It is +名词+从句 It is a fact that… 事实是…… It is good news that … ……是好消息 It is a question that … ……是个问题 It is common knowledge that … ……是常识 类似的名词还有：a pity,a wonder,surprise,no wonder等.
(3) It is +过去分词+从句 It is said that … 据说…… It is reported that … 据报道…… It has been proved that … 已证明…… It must be proved that… 必须指出…… 类似的过去分词还有：known; estimated; expected; believed; thought; hoped; noted; discussed; required; decided; suggested; demanded; made clear; found out,etc. (4) It +不及物动词+从句 It seems that … 好像是…… It happened that… 碰巧……
判断是否是强调句有一个方法，就是将that以后的“句子的其余部分” 拿出来单独看，看有没有缺成分，缺的是什么成分，再把谓语动词后 面that前面的那部分，带到你认为缺成分的地方，如果放进去是一句 完整的句子了，那就说明是强调句。）
It was how t foreign languages __B___attracted the audience's interest.
英语各种从句的用法
吴诗君 曹璐 杨青慧 李双 蒋亚萍 王搏怀
主语从句 表语从句
从句
定语从句 同位语从句
宾语从句
状语从句
Subject Clause(主语从句)
定义：在句中作主语的句子叫主语从句 1.由从属连词that，whether，if引导；
That the plates are moving is beyond dispute. Whether he will come or not is still a question.
类似的形容词还有：strange; natural; obvious; true; good; wonderful; possible; unlikely; quite; unusual; certain; evident; worth-while; surprising; interesting; astonishing, etc.
The problem is when we can get a pay rise.
结构：主语 + 连系动词(be,seem,look) + 句子作表语 (1) 从属连词that，whether, as, as if 等；
It follows that … 由此可见……
It has turned out that … 结果是……
类似的不及物动词还有：seem, appear, occur, follow, happen,turn out, etc.
(由it作形式主语和it引导强调句的比较：
it作形式主语的结构It is+名词/形容词/过去分词+主语从句；而强调 句则不同，它的结构是：It+be+被强调部分+that+从句