as定语从句用法总结：
as的含义很多。它可用作关系代词，引导定语从句。主要分为两种情况：
1. 引导非限制性定语从句
as引导非限定性定语从句时，用以指代整个句子的内容，可放在主句之前或主句之后，也可放在句中。as在从句中可以充当主语、宾语，往往有“正如、正像”之义。
多见于一些固定结构中，如：as we know, as is known to all, as you can see/ imagine/ expect, as is expected, as is often the case等。
As we know, China is becoming more and more powerful in the world.
众所周知，中国在世界上正变得越来越强大。(as在从句中作know的宾语，指代整个主句的内容)
He was late for work this morning, as is often the case.
今天早上他迟到了，他常这样。(as在从句中作主语，指代整个主句的内容)
He, as you can see, is one of the most hard-working teachers in our school.
正如你看到的，他是我们学校最勤奋的老师之一。(as引导的从句放在中间)
2. 用于固定结构中
as引导限定性定语从句，还用在such/so...as（正如）和the same as（正像）的结构中，可指代表人或物的先行词。
I have had such an unforgettable spring festival as they have.
=I have had so unforgettable a spring festival as they have.
我过了一个和他们一样难忘的春节。（as指代先行词spring festival,在从句中作have的宾语）
No teacher will object to such a rule as prevents students from bringing mobile phones to school.
没有老师会反对这项阻止学生带手机进校的规定。（as指代先行词rule,在从句中作主语）
I have never seen such a beautiful girl as she is.
我从来没有见过像她这样漂亮的女孩。（as指代girl,作is的表语。as she is= like her）
I bought the same pen as you did.
我买了一支像你买的一样的钢笔。（as指代pen,作宾语）