关系代词as 引导的定语从句
1、as 可以在定语从句中做主语或宾语，代表前面整个句子。

如：
He married her, as was natural.
他跟她结婚了，这是很自然的事。

He is honest, as we can see.
他很诚实，这一点我们看得出来。

2、as 引导的非限制性定语从句可以放在主句之前、主句之后，甚至还可以分割主句。

另外，as 常
常有“正如、正像”的含义。

如：
As is known to all, China is a developing country.
众所周知，中国是发展中国家。

He is from the south, as we can know from his accent.
他是南方人，从他的口音我们就可以知道。

John, as you know, is a famous writer.
正如你所知，约翰是个著名作家。

3、当先行词受such, the same 修饰时，关系词常用as。

如：
I’ve never heard such stories as he tells.
我从未听过象他讲的这样的故事。

He is not such a fool as he looks.
他可不象他看上去的那样傻。

This is the same dictionary as I lost last week.
这部词典跟我上星期丢失的一样。

注意：
当先行词受the same 修饰时，偶尔也用 that引导定语从句，但与as引导的定语从句意思有区别。

如：She wore the same dress that she wore at Mary's wedding.
她穿着她在玛丽婚礼上穿过的那个同一条连衣裙。

She wore the same dress as her younger sister wore.
她穿就像着跟她妹妹所穿的一样的连衣裙。