有读者问： 为什么可以说：
(1)He married her, as was natural.
却不可以说：
(2)He married her, as was disgraceful.
而只能说：
(3)He married her, which was disgraceful.
这是一个有趣而同时也很重要的问题。许多语法书告诉我们说，as 可以用作关系代词，其作用等同于 which。 但这并不是说关系代词 as 与关系代词 which 完全一样，在任何情况下都可以互换使用。实际情况是：二者有时可以互换，有时则不可互换。不论是哪一种情况，其范围都很广。我们在此只能谈谈上面提问中所涉及的问题。
上面所引的例（1）之类与例（3）之类中的 as 与 which 有两点相同之处： 一是二者都引导一个具有评述性的非限制性定语从句；二是二者都修饰其前的整个陈述句。二者的不同之点在于例（1）之类可用 as， 一般亦可用 which; 例（3）之类则只能用 which, 不能用 as。
让我们先讨论例（1）之类可用as 亦可用 which 的问题。一般说来，这二者的确是可以互换使用的，如：
(4)He was late for school, as （which ）was usual with him.
(5)She is extremely popular among students, as （which ）is common knowledge.
但它们的含义并不相同。其主要区别在于：
1. as 具有“显然”或“为人所熟知”的含义，而 which 则无此含义。
2. as 从句仅仅提供一种附加说明，而 which 从句则提供较为重要的评述。
3. as 具有连词的性质（有一位语法大师称之为“关系连词”），其后往往可插入it而不影响整个as从句的意义，如我们常说的 as is often the case, as is usual with him 等， 而 which 则无连词的性质，其后也不可插入 it。
4. as 从句可位于句首，如：
(6)As is often the case, she was the first to come to class.
(7)As was the custom with him, he went out for a walk.
which 从句则不可置于句首。
5. 有时习惯上常用 as 而不用 which, 上面所引例 （1）中的 as was natural 即是一例，再如：
(8)Mr. Smith has been with us many years, as is well known .
同样 as 用作宾语时，这种习惯用法也似乎不少，如：
(9)Mr., Smith, as we know , works long hours.
现在让我们再讨论上述例 （3）之类的问题，即为什么有时只可用 which 而不可用 as 的问题。这个问题，较之于上面有关例（1）之类的问题，就简单多了。其原因可以说只有一个，那就是 as 从句须与其前的主语在意义上和谐一致（这可能与as 的本义“如同”有关）， 而which 从句则无此限制。具体到上述例（3），其中的 disgraceful —词显然与主句的意义不和谐一致，所以只可说 which was disgraceful, 而不可说 as was disgraceful, 而这也正是上述例（2）的错误之处。由于同样的原因，下面一句亦须用 which 而不可用 as:
(10)He married her, which was unexpected. （如用 expected 则应用 as ）
最后请注意， as 在其所引导的非限制定语从句中用作主语时，其后除可用 be 外，还可用其他连系动词，如：
(11)She has married again, as seemed natural.
(12)Molly said nothing, as became her age and position.
在这种 as 从句中，除可用 be 和别的连系动词之外，其他行为动词一般都不可用，如下面一句只可用 which 而不可用 as:
(13)She has married again, which delighted us.
但在 as 之后可插入 it 的情况下则属例外，如下面一句中的 as 之后则可用行为动词 happen:
(14)The boy was run over by a motor car, as often happend here in New York.
