（1）在as引导限定性定语从句时，只是用在一些固定的结构中，即只能用在such，the same，as，so的后面。
a.He'11 repeat such points as are discussed in the book.
b. Such a student as works hard will be sure to succeed.
c.He tried to make as few mistakes as he could avoid.
d.He is not the same playboy as we knew.
e.The instrument is not such an instrument as I saw on the exhibition.
f.He is not such a man_________would leave his work half done.
A. who
B. whom
C. as
D. that
正确答案：C。
（2） as引导非限定性定语从句可位于句首，而which不能这样用。
a.As is known, the whale is not fish but a mammal.
众所周知，鲸不是鱼类，而是哺乳类动物。（as指的是the whale is not fish but a mammal整个句子。）
b. _________is known to the world, Mark Twain is a great American writer.
A. That
B. Which
C. As
D. It
正确答案：C。
c. _________might be expected, the response to the question was very mixed.
A. As
B. That
C. Which
D. What
正确答案：A。
（3） as和which都可引导非限定性定语从句，代替前面整个句子。
a.He married her, as/which was natural.
b.He was late for school, which/as is often the case.
但因为as引导的定语从句有“正好……”，“就像……”，“由……而知”的含义，所以，当主句和从句在内容上一致时，或者说从句内容是顺着主句意思说下来时，关系词要用as。若主句和从句内容上不一致，或从句的内容对主句的内容起反对、排斥、否定等消极的作用时，则多用which。试分析比较以下例句：

 
她很细心，这从她的工作中就可以看出来。