英语语法问题

Question

it has been estimated that= what has been estimated is that=as has been estimated ，
为什么可以这么替换，其中的语法是什么

Answer 1

it has been estimated that...
it 是形式主语，that 从句是真正主语。that 从句可以移到句子开头，变为主语从句 That ...has been estimated.

what has been estimated is that...
其中的关系是what = that...。把what 和 is 去掉，也能还原成 That...has been estimated.

as has been estimated， 【estimated 后面有逗号隔开】
as 作关系代词，引导非限制性定语从句，先行词是整个主句，即逗号后面的句子。

例子：
It has been estimated that around a third of China's carbon emissions in 2005 were due to manufacturing exported goods. （据估计，2005年中国大约有1/3的碳排放是由于生产出口产品造成的。）
= What has been estimated is that around a third of China's carbon emissions in 2005 were due to manufacturing exported goods.
= As has been estimated, around a third of China's carbon emissions in 2005 were due to manufacturing exported goods.
= That around a third of China's carbon emissions in 2005 were due to manufacturing exported goods has been estimated. （这个结构不平衡）

Answer 2

it has been estimated that= what has been estimated is that=as has been estimated ，
可以这么替换，因为 意思一样 只不过 结构不同
1 that从句 是主语从句 作真主语
2 that从句 是表语从句中 说明 主语 what has been estimated
3 as 是关系代词 代替 后面主句所要表达的内容 引导非限制性 定语从句 并且在定从中 作主语本回答被网友采纳

Answer 3

It 做形式主语，后面that引导的从句是真正的主语，that是不可省略的。这种一般用于主语比较长的时候，为了避免头重脚轻的感觉，方便听的人理解。
What 引导的是主语从句，并且在主句中做的主语。而后面的That 引导的从句是真正的主语，切that在引导主语从句是也是不可省略的。
As has been ... As 引导的是非限制性定语从句，举个最常见的例子，

It has been known to all that...=what has been know to all is that..= As has known to all,...

英语中这种句子结构的转换一般用于写作，不同的结构表达同一个意思，不会让读者觉得文章无聊，通篇一种表达，像流水账。