1 我们知道。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。除了名词性关系分句不能采用外置主语从句结构外，其它三种名词从句都可以采用外置结构。

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 doesn't 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 采用外置主语结构的目的是避免头重脚轻，因此，主句结构通常比较简单，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。例如：

It doesn't matter who said that. 主句为SV结构。

It embarrassed her that he was so angry. 主句为SVO结构。

It suddenly occurred to her that Joe was afraid of being alone. 主句为SVA结构

3 例句分析

It depends on the weather whether we will go.

二个相同的发音读起来拗口，本句不宜采用外置结构。应改为：

Whether we will 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(不知道这个是时间状语从句还是主语从句？)

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。