英语大佬帮我分析一下下面这个句子的语法

Question

英语大佬帮我分析一下下面这个句子的语法有一个病句:It is know to all，Taiwan belongs to China改法是把It改成as虽然平时看文章都是这么写的，可是我不能理解里面的语法，我刚刚整理试卷的时候笔记写的是引导主语从句无引导词且位于句首不能用which句子做先行词，不懂诶然后刚刚百度连词的时候我看见了一个类似的句子[误] Which you can see,he is always ready to help others.

[正] As you can see,he is always ready to help others.

[析]as引导非限制性定语从句，当"正像"讲。但是这个又说是引导非限制性定语从句
这是为什么呢？？

Answer 1

这里的as是关系代词
as引出非限定性定语从句时，代替整个主句，对其进行说明。但通常用于：（as we all know, as it is known, as is known to all, as it is, as is said above, as always mentioned above, as is usual, as is often the case, as is reported in the newspaper) .
as在非限定性定语从句中作主语、表语或宾语，且引出的从句位置比较灵活，可位于句首或句末，也可置于主句中间。通常均由逗号将其与主句隔开。
as有“正如……， 就像……”之意。追问

那为什么不能用it啊

追答

It is know to all that Taiwan belongs to China.

it形式主语，that从句是真正主语

缺少that，只能把it改为as,成为非限定性定语从句。