1 as (引导从句,对前述内容作补充)本情况,该事实
She has married again, as was expected.
她已再婚,这是意料中的事。
We are tired, as anyone can see.
我们累了,这是有目共睹的。
2 主语从句
3 表语从句

奇怪，看到英语首先要联想到是虾米从句，懂了都不行吗？

第一个as是代词

回复：4楼
化验过？

1。As is known to us all, the earth is round.
as引导定语从句，as在从句中充当主语。
2. It is known to us all that the earthe is round.  
It充当形式主语，真正的主语是that the earthe is round.  
3.what is known to us all is that the whale is not fish but mammal.
what 引导主语从句，that引导表语从句。

6楼的回答更完整

。As is known to us all, the earth is round.
2. It is known to us all that the earthe is round.  
3.what is known to us all is that the whale is not fish but mammal
1所知，我们所有人，地球是圆的。
2.众所周知，earthe 是倒圆角。
3.什麼大家都知道是鲸鱼不是鱼，但哺乳动物

As is known to us all。。。非限制定语从句。。

谢谢楼上的各位朋友！

As is known to us all, the earth is round.
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楼主的句子有点生硬别扭。英语中当然有as is等用法。它是一个由连接词as引出的句子成
分而已，等于"...and so is "(如果你要另起一个独立的句子，就是so is）。as is 的大致用
法应该是这样的，比如：Chinese people are very straightforward as are the Americans, of course.

都挺古老了，至少不简洁，阅读时遇到的频度低。
so what? now what? then what ?