在初级英语阶段，学生知道引导定语从句的关系代词有who (whom, whose),that和which等。到了中高级阶段尤其是在考研英语的复习阶段，这些显然是不够的。还应了解but和than也可以用作关系代词的情况，提高语言识别和理解能力，这对考生在考试时有着很重要的提示作用。
一、but
but在英语句子中可用作关系代词，引导定语从句。但应特别注意but虽然形式上是肯定的，意义上却是否定的。这正是but和其它关系代词(who, that , which等)不同之处。例如：
(1)There is no rule in English but has exceptions.英语中没有无例外的规则。
(2)There is no old habit but may be cured by a strong will-power.通过强大的意志力没有改不了的旧习惯。
(3)With the introduction of the electronic computer, there is no complicated problem but can be solved in a few hours.由于引进了电子计算机，没有在几小时内解决不了的复杂问题。
(4)There is no human being but errs.没有不犯错误的人。
如果将以上例句(1)、(2)和(3)中的关系代词but改成that，或者将例句(4)中的关系代词but改成that或who,那么定语从句必须改成否定从句才能与原句意义相一致。例如：
(5)There is no rule in English that has no exceptions/ that has not any exceptions.
(6)There is no old habit that may not be cured by a strong will-power.
(7)With the introduction of the electronic computer, there is no complicated problem that cannot be solved in a few hours.
(8)There is no human being that(who)makes no mistakes/that(who)does not makes any mistakes.
【注意】如果(5)、(6)、(7)、(8)中没有明确的否定词no或not any等，其意义则完全相反。
二、than
than也可以用作关系代词，引导定语从句，但要注意主句中必须有比较级字样和than前后呼应。例如：
(9)He smoked more (fewer) cigarettes than (he bought.) were normally available.他吸的香烟比他买的(弄到的)要多(少)。这里than在语法功能上相当于that/which的功能，但是这里不能改成后者，因为主句中有比较级more。
请再看下列的例句：
(10)We often advise him not to drink more wine than is good for his health. 我们经常劝他为了身体健康不要喝过量的酒。
(11)He often sat in a small bar drinking considerably more wine than was good for his health.他经常坐在小酒馆里，喝过量的酒。这不利于他的身体健康。
(12)To understand the situation completely requires more thought than has been given thus far.要彻底理解这种局势需要作比迄今为止更多的思考。
如果我们仔细观察一下，就会发现，例句(9)中的名词cigarettes用的是复数形式，所以从句就用了复数形式的动词were,而在例句(10)、(11)和(12)中由于所用从句中便分别用了单数形式的动词is、was以及has been given。由此可以证明，以上各例句中的than从语法功能来看是关系代词，所引导的从句是定语从句，而不是连接词引导的状语从句。
