方法一：看指人还是物

这个方法的意思是：指人时用who（作主语）、whom(作宾语)和whose(作定语，既指人又指物)；that既可指人又可指物，但在当代英语中多指物；which只能指物，that与which在句中作主语或宾语；在非正式文体中，作宾语的whom,which或that可省略。如：

In the beginning, people who bought them often got angry. (who指人，修饰先行词people,作从句的主语。)刚开始，买的人经常会生气。

They needed a plant which didn't need as much water as rice.(which指物，修饰先行词plant,作从句的主语，此处which可换为that。)他们需要一种不像水稻那样需要大量水的作物。

I've bought a new dictionary whose cover is blue.（这里whose指物，在从句中作定语。）我买了一本封皮是蓝色的字典。

难点：

（1）whose既可指“某人的”，也可指“某物的”，指物时不可想当然误用which,但可使用“the+n.+of which”的表达方法，也可用“of which +n”的表达方式。

①I've bought a new dictionary, the cover of which is blue.

②I've bouth a new dictionary of which the cover is blue.

(2)在非正式文体中，当引导词who位于从句句首时，可替换whom作宾语。

如：Do you know the boy who/whom we talked to just now?

方法二：看介词在何处

当介词置于从句之首时，若指人，介词之后只能用whom而不能用who或that;若指物,则只能用which而不能用that。试比较以下两组句子：

1Do you know the woman with whom I shook hands at the school gate?(此处介词with之后的whom不可替换为who或that,也不能省略。)

Do you know the woman whom I shook hands with at the school gate?(此句中的引导词whom作介词with的宾语，可换为who或that,也不能省略。)

难点：（1）当who, whom, which, that等引导词在从句中作宾语而省略时，从句中不能再加宾语。如：This is the pen she bought it yesterday.(此处的it应去悼，因为作宾语的which或that已省略。)

（2）两个以上的简单句组成一个句子时，必须用关联词来连接两个句子，而不能用代词。如：This is our monitor, without him, we couldn't win the game.(此句中的人称代词him必须改为引导词whom。)

Can you see the tall building, in front of it there a large play ground?(此句中的代词it 必须改为引导词which。)

如何选用定语从句中的引导词，关键是要看引导词在从句中作什么成分。若作主语，则在who, that或which之中选择；若作定语，则用whose;若作状语，则在when,where,why之中选择。例如：

1. Have you ever been to the place which/that we visted last year 你去过我们去年参观的那个地方吗?

Have you ever been to the place where we worked last year? 你去过我们去年工作的那个地方吗？

第一句中的引导词which/that作及物动词visited的宾语，可省略，不可换用where;第二句中的引导词where作不及物动词worked的状语，不可换用which或that,但可用in which来替换。

2.Do you still remember the day which/that we spent together? 你还记得我们一起度过的那一天吗？

Do you still remember the day when we stayed together? 你还记得我们呆在一起的那一天吗？

以上第一句中的引导词which/that作及物动词spent的宾语，可省略，不可用when替换；而第二句的when作不及物动词stayed的状语，不可换用which或that,但可用on which来替换。

难点：要把握句子结构，正确分析句子成分，切不可见到表示时间的名词就用when,见到表示地点的句词变用where。

定语从句引导词的使用还要注意一些特殊情况,如who 与that均可指人,which和that都可指物,但这些引导词在具体的语言环境中有区别,不能换用。如：

This is the first time that I’ve been to the museum, which I have been looking forward to for many years.这是我第一次去博物馆，我盼了好多年了。

当先行词被序数词修饰时，引导词只用that而不能用which；在非限制性定语从句中，只能用which而不能用that,故此句中的that与which不能互换。

which is the book that you want to borrow? 你想借那一本书？

当先行词是which时，引导词用that。

另外一种常考的点是way和reason引导的定语从句。如：I dislike the way in which/that/× he talks to his mother. 我不喜欢他和他母亲说话的方式。

此句中way在定语从句中充当方式状语，引导词用in which that 或者省略。