（1） 当被修饰的是整个句子时，只能由as或which引导非限制性定语从句。

（2） as引导的非限制性定语从句，位置比较灵活，可放在主句前、主句后，也可插在主句中；而which引导的非限制性定语从句只能放在主句之后。

（3） as常译成"正如...，像..."；which常译成"这就使得...，这一点"。as通常用于：

　　as has been said before 如上所述

　　as may be imagined 正如可以想象出来的那样

　　as is known to all / as we all know 众所周知

　　as was expected 正如预料的那样

　　as has been already pointed out 正如已经指出的那样

　　as we all can see 正如我们都会看到的那样

6． 只能用that引导的场合

1. 先行词是 all, few, little, much, something, everything, anything, nothing等不定代词时

All that we have to do is to practise every day.

2. 先行词是序数词或形容词最高级，或被序数词或形容词最高级所修饰时

The first lesson that I learnt will never be forgotten.

3. 先行词被all, any, every, few, little, no, some等词修饰时

I have read all the books that you gave me.

4. 先行词被the only, the very, the same, the last所修饰时

He is the very person that I'm waiting for.

5. 当先行词既包括人又包括物时

He talked about the men and the books that attracted him.

6. 当主句是以who或which引起的特殊疑问句时，为了避免重复，常用that代替关系词 who或which。

Who is the person that is standing at the gate？

四、 "介词＋关系代词"用法

1. 介词的确定应依据定语从句中短语的习惯性搭配

eg：Who is the girl with whom you just shook hands? (shake hands with...是习惯性搭配)

2. 介词常受先行词的制约（即介词和先行词的搭配）

eg：He built a telescope through which he could study the skies.

(through which 即through the telescope)

3. 当关系代词作"动词＋介词"固定短语的宾语时，此时含有介词的动词短语一般不拆开，介词仍放在动词的后面。

eg：This is the watch which you're looking for.

4. 当介词放在关系代词前面时，关系代词不能省略。介词后不能用关系代词that.

eg: This is the school at which I used to teach.

5. 表示所属关系时，介词用of, 构成"the +名词+of which / whom"或"of which / whom+the +名词"的形式，相当于"whose +名词"。

eg: I live in the room, the window of which / of which the window / whose window is open.

五、 关系代词as引导的限制性定语从句

"the same＋名词"，"such＋名词"，后通常用as引导定语从句。

the same＋名词 as... 与...一样 （同一类事物）