is not clear to anyone. 这件事怎样发生的，谁也不清楚。
Where Coulomb confirmed the inverse-square law for the electrostatic force remains to be proved. 库仑到底是在哪里确定静电力的平方反比定律还有待证实。
5) 关系代词what引导
What we need is a precision tool. 我们需要的是精密工具。
What the physicists were waiting for at that time was a solar eclipse. 那时，物理学家们等待的就是一次日食。
What make the lab more impressive are the modern instruments. 为实验室增色的是现代化的仪器设备。
10.1.2 主语从句与形式主语it
有时为了考虑句子平衡，通常在主语从句的位置使用形式主语it，而将真正的主语从句移至句末。用 it 作形式主语的 that 从句有以下四种不同的搭配关系:
"It+be+形容词+that 从句”，如:
It is necessary that ... 有必要……
It is natural that... 很自然……
It is strange that... 奇怪的是……
"It+be+过去分词+that从句”，如:
It is known to all that ... 众所周知……
It is reported that... 据报道……
It is said that... 据说……
It has been proved that... 已证实……
"It+be+名词+that 从句”，如:
It is a fact that ... 事实是……
It is an honor that... 非常荣幸……
It is common knowledge that... ……是常识
"It+不及物动词+that 从句”，如:
It appears that ... 似乎……
It seems that... 似乎……
It happened that... 碰巧……
It occurs that... 碰巧……
用it作形式主语时分以下三种情况：
1) 对于以连词that引导的主语从句，通常用形式主语代替主语从句:
For example, in electric circuits it is often assumed that the relations between voltages and currents are linear. 例如，在电路中通常假设电压和电流的关系是线性的。
It has been found repeatedly in science that the discovery that two branches are related leads to each branch helping in the development of the other. 在科学中经常发现，与两个分支相关的发现会导致两方相互促进发展。
It is quite clear that the whole project is doomed to failure. 很清楚，整个计划注定要失败。
It's a pity that he didn't find any useful results in the experiment. 很遗憾他在实验中没找到什么有用的结果。
It is important that the experiment should be carried o
ut in vacuum. 实验必须在真空内进行。
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替主语从句，也可直接在句首使用主语从句:
Whether they would support us was a problem. 他们是否会支持我们还是一个