*放在一个名词后修饰这个名词,充当这个名词的定语的句子叫定语从句,引导定语从句的词叫关系代词/关系副词;被定语从句修饰的名词叫先行词.关系代词在定语从句中充当宾语时可省略.

*起名词作用的从句叫名词性从句, 引导名词性从句的词叫连接代词/连接副词. 名词性从句又分主语从句、宾语从句、表语从句、同位语从句.

*起副词作用, 充当状语的从句叫状语从句. 引导状语从句的词叫从属连词.

关系代词：that , which , who , whom , whose , as

关系副词：when , where , how , why

连接代词：that , which , who , whom , whose , what

连接副词：when , where , how , why

从属连词：if , when , as , unless , since , after , before , until等

一.that与which在引导定语从句时的差别：

   (1) that可指人,也可指物;而which只能指物;

   (2) that 只能引导限制性定语从句,而which既可引导限制性定语从句,又可引导非限制性定语从句

(3) 介词后只能用which,不能用that.

Note：在定语从句中,含有介词的短语动词(如：look for , join in , go in for , look after ),一般不可拆开

     如：Is this the pen which you are looking for?

     不可说：Is this the pen for which you are looking?

(4)当先行词为anything , nothing , everything , all , much , few , little等不定代词时,其后的定语从句只能由that引导, 不可用which.

(5)当先行词前面有序数词或最高级修饰时,其后的关系代词只能用that.

(6)当先行词被the only ,the very , the last 修饰时, 其后的关系代词只能用that.

(7)当主句是以who或which开头的特殊疑问句时,定语从句要用that引导, 而不用which或who(whom).

(8)当几个先行词并列,既有表示人的,又有表示物的,这时定语从句要用that而不用who(whom)或which引导.

二. that与what

(1)what只能引导名词性从句, 而that既能引导名词性从句,又可引导定语从句.

(2)that与what在引导名词性从句时的差别：what在从句中充当成分, 而that不充当成分,只起连接作用.

*引导宾语从句的that可省略,而引导主语从句、表语从句及同位语从句的that不可省略

三. 当先行词是表地点的名词时,其后的定语从句不一定都能用where引导,这要看先行词在定语从句中逻辑上充当什么成分.如果充当地点状语, 定语从句应用where引导; 如果充当主语、宾语,就应用that或which引导.

    当先行词是表时间的名词时, 其后的定语从句不一定都能用when引导,其道理同上

四.可用来代指整个主句所表达的意思,译为“这一点”的关系代词有which和as,其差别是: as能转译为“正如”, which不能,所以用as引导定语从句时,定语从句的谓语动词往往是say , imagine , know , expect , see , report等,且as引导的定语从句可放句首或句末,而which引导的定语从句只能放句末,不能放句首.

五.as引导定语从句时,也可只修饰一个名词,这时前面肯定有as , so , the same , such等词语与之相呼应,以构成固定句型as…as … ,so… as… , the same …as…(换句话说,就是当先行词前面有so , as , the same 这样的修饰词时,其后的定语从句只能用as引导,不能用that , which或who等)