夸克《英语语法大全》中对 as 引导定语从句和状语从句的论述有以下内容（中译版第1546页；隶属于15.55节）：

as 作关系词时，引导一种句子性关系分句。这种关系分句可以在先行词（在这种情况下是它所依附的分句或句子）之前或插在其间。这个as-分句位置的灵活性介于关系结构和状语结构之间，像句子关系词 which 一样，as 可作关系代词：

She is extremely popular among students, as is common knowledge. (比较: which is common knowledge)

I live a long way from work, as you know. (比较: which you know)

带有关系词 as 的分句还有 as everybody knows, as you may remember, as you say, as I can see, as I have said, as I’m told, as you may have heard。

as 作从属连词显然是引导一个状语分句的，而且句子先行词为 it 所代替。as 与 insofar as [“to the extent that”]大致同义：

He is the best candidate, as it seems. [“…insofar as it seems that he is the best candidate.”]

带有从属连词 as 的分句还有 as it appears, as it happens, as it transpired, as it may interest you to know, as I see it, as I interpret it。

我对上面夸克《英语语法大全》中对 as 引导定语从句和状语从句的论述没看懂，为什么 as everybody knows 等是定语从句，而 as it appears 又是状语从句呢？