1、关系代词的形式与用法
主要的关系代词有 that, who, whom, whose, which, as 等。其中who, whom 只用于指人，which, as 只用于指物，whose, that 既可于指人也可指物。关系代词在定语从句主要用作主语、宾语、表语或定语。如：
People who smoke cannot travel in certain parts of the train. 吸烟的人不能在火车的某些地方走动。
He is not a person whose promise you can build on.他不是那种你可以信赖他诺言的人。
They read us the stories which they themselves had written.他们给我们读他们自己写的故事。
2、that 与 which的用法辨析
两者都可指物，常可互换，但引导非限制性定语从句或直接放在介词后作宾语时，通常只用which；当先行词是much, little, none, all, few, every(thing), any(thing), no(thing) 等不定代词或受其修饰时，通常用that，另外，当先行词受the very, the only, the same 以及最高级和或序数词(包括last, next等)等修饰时，通常都只用that。如：
He invited us to dinner, which was very kind of him. 他请我们去吃饭，这是他的好心。
He grasped at anything that might help him. 他试图抓住任何对自己有利的东西。
There was little that the enemy could do but surrender. 敌人无法，只好投降。
This is the only reference book (that)I find useful. 这是我发现的唯一有用的参考书。
注：当关系代词在定语从句中用作表语时，通常用that，另外当先行词是一个既指人又指物的并列词组时，通常用that。
3、as与which的用法辨析
引导限制性定语从句时，在such, as, the same后只能用as，其他情况用 which；引导非限制性定语从句时，有时两者可互换，但若位于主句之前，则只能用 as。如：
He is not such a fool as he looks. 他并不像他看起来那么傻。
You must show my wife the same respect as you show me. 你必须像敬重我一样来敬重我妻子。
As is known to all, smoking is harmful to one’s health. 众所周知，吸烟有害健康。
注：as 引导的非限制性定语从句应与主句在意义上和谐一致，which无此限制。如：
He married Mary, as [which] was expected. 他娶了玛丽，这是大家预料到的。
He married Mary, which was unexpected. 他娶了玛丽，这让大家感到很意外。(不用as)