曹老师你好，关于主语从句中it做形式主语的“主语从句”我有两个疑问，烦请你帮忙解答一下，谢谢你。
问题背景：
在学习主语从句时，遇见了it做形式主语的句子，并且各种市面的教学资料都是给出了“4种常见的句型”，分别是：
It+ is（系动词）+adj+ that从句
It+ is（系动词）+ n + that从句
It+不及物动词 + that从句
It+ is（系动词）+过去分词+ that从句
我的疑问是如下：
1. 列举这四类it做形式主语的主语从句句型基本都是“主系表”结构，然后主语后置。（这个我理解，由于主系表确实导致了主语从句“头重脚轻”的感觉）
但不了解“主谓”和“主谓宾”等剩下句型结构是不是也能转化为it作形式主语的主语从句？
2. 列举的这四类it做形式主语的主语从句中如何基本都是从属连词that引导的“陈述句”后置，那样像whether作为从属连词，能否也进行这样的转换，那样还有连接代词what..等，与连接副词when…等，是不是也可以用it做形式主语来进行变换？
1 大家了解。名词性从句有四种：that从句（陈述分句）、疑问分句、感叹分句、名词性关系分句。除去名词性关系分句不可以使用外置主语从句结构外，其它三种名词从句都可以使用外置结构。
1）That从句作外置主语从句（略）
2）疑问分句作外置主语从句：
It is unclear why/when she did that.
It doesnt matter who said that.
It remains a question whether he will come or not.
3) 感叹分句作外置主语从句：
It is incredible what a fuss some people make.
2 使用外置主语结构的目的是防止头重脚轻，因此，主句结构一般比较简单，多为主系表结构，但又不限于此。主句为SV或SVO结构也完全可以。比如：
It doesnt matter who said that. 主句为SV结构。
It embarrassed her that he was so angry. 主句为SVO结构。
Itsuddenlyoccurredto her thatJoewasafraidof being alone. 主句为SVA结构
3 例句剖析
It depends on the weather whether we will go.
二个相同的发音读起来拗口，本句不适合使用外置结构。应改为：
Whether wewill go depends on the weather.
It made us very happy that she was saved.
正确
It has been decided yet when they will start the project.
主句结构错误。应改为：
It is yet to be decided when they will start the project.
It hasn’t been decided who will be sent to work there.
正确
It was clear why he had asked for a conference.
正确
It doesn’t matter whether one is wealthy or not.
正确
It makes no difference whether one is wealthy or not.
正确
It was 1949 when I was born
本句中的when从句为非限制性定语从句。
When I was born was 1949. 句子不通。
