4.作主语时，which既可以作系动词be的主语，也可以作实义动词的主语；而这时as 只可以做系动词be的主语。例如：
A） He married her, which was natural.（可用as代替which）他和她结婚，这是很自然的事。
B） He won a/the Nobel Prize in chemistry, which made him illustrious. （不可用as 代替which）他获得了诺贝尔化学奖，这使他名扬天下。
5.当定语从句位于主句之后，纯粹表示主句所述内容，不带有“如……那样”的意思时，往往用which,而不用as。例如：
A） They were invited to the state banquet, which（as）was a great honor to them.他们被邀请参加国宴，这对他们来说是莫大的荣幸。
B） We had to sleep in our wet clothes, which was most uncomfortable.我们不得不穿着湿衣服睡觉，这简直太不舒服了。
C） Mummy always treats me just like a baby, which I can't bear.妈妈老把我当成小孩对待，
这让我无法忍受。
6.当定语从句有“如同……那样”含义时，用as比用which更常见，而这些从句几乎成为固定说法。例如：
A） Things will turn out contrary to one's wishes, as is often the case.事与愿违，这是常用的事。
B） As was natural, this inordinate hope was followed by an excessive depression.这种过分希望之后，接着是极度的沮丧，是很自然的事。
C） Chaucer is buried in “poet's Corner”，as might have been expected.正如人们已经预料的，乔叟被葬在“诗人角”。
D） The material is elastic, as is shown in the figure.这种霉烂有弹性，如图所示。
E） As we know（众所周知）
F） As has been said above/before（正如前文所述）
G） As has been pointed out（正如已经指出的）
H） As might be imagined（可以想象得到）
7.当定语从句的关系代词指的是先行词本身时，只能用which。例如：
A） These apple trees, which I planted three years ago, have not borne any fruit.这些苹果树是我三年前种下的，没结出任何果实。
B） The Thames, which is now clean enough to swim in, was polluted for over a hundred years.泰晤士河，现在已经干净，可以在里面游泳，它曾被污染了一百多年。
8.带介词的典型的定语从句，必须用which，不能用as。例如：
A） There is a great deal of oxygen around us, without which we could not live.我们周围有许多氧气，没有它我们就无法生存。
B） The shed in our garden, in which we often played, has lasted for a long time.我们经常玩耍的花园里的那个棚子，已经有很长时间了。