定语从句的详细解析：
1.什么是定语从句？
要理解定语从句，必须先理解定语。
1）何为定语
定语：说明人或事物是什么样子的（起形容词的作用）
EX1: They are interesting books.
[定语interesting: 说明 books是有趣的]

EX2: I have a blue bag. 
[定语blue，修饰bag，说明它是蓝色的]

EX3:I have something important to tell you. 
[定语important，修饰something，说明事情是重要的]

定语可修饰：名词（例子EX1和EX2）和代词（例子EX3）
 
定语可由句子来担任：
EX4: Jack is reading the book that I gave him.
杰克正在读那本我送给他的书。
[that I gave him是完整句子结构，作定语，修饰the book]
[ 句子作定语 → 定语从句 ]


2）何为定语从句
-定语从句：从句作定语， 修饰名词或代词
-先行词：被定语从句修饰的词
-关系词：引导定语从句的词

Jack is reading the book  that I gave him.
[ 先行词: the book ]      
[ 定语从句:that I gave him]
[ 关系词：that）

The man who called yesterday is here now. 
昨天打电话的那个人现在在这里。
[ 先行词: The man ]      
[ 定语从句: who called yesterday]
[ 关系词：who）

2.关系词that/which/who用法
1)that/which/who用法一：


注：that更常用于口语，which/who更常用于书面语。

例析：
Do you know the boy  that  is standing over there? 
[先行词the boy是人,关系词可用that]
你认识站在那里的那个男孩子吗？

Have you seen the movie  that  we talked about? 
[先行词the movie是物,关系词可用that或which]
你看了我们谈论过的那部电影吗？

I saw the letter  which came yesterday. 
[先行词the letter是物,关系词可用which或that]                   
我看到了昨天寄到的那封信。

The man  who was driving a red car  is my brother. 
[先行词the man是人,关系词可用who 或that]
开着红色汽车的那个人是我的兄弟。

2)that/which/who用法二：


（相关链接：主语是什么  宾语是什么）
Do you know the boy  that/who is standing over there?
[从句意指: the boy is standing over there]
[that/who指代the boy，在从句中作主语]

I saw the letter  which/that came yesterday. 
[从句意指: the letter came yesterday]
[that/which指代the letter，在从句中作主语]

Have you seen the movie  that/which we talked about? 
[从句意指: we talked about the movie]
[that/which指代the movie，在从句中作talked about的宾语]

(我刚遇见的那个女孩非常友好。)
The girl  that/who I just met  was very friendly.
[从句意指: I just met the girl]
[that/who指代the girl，在从句中作met的宾语]

TIPS:
-that/which/who在定语从句中有时可省略，详情请点击：
定语从句that which省略
-虽然在多数情况下，which和who可与that换用，但有些情况只能用that，详见链接：
定语从句只能用that的情况

3.定语从句其它关系词用法（基础阶段仅作了解即可）


She is the woman whose car was stolen. 
她就是那个车被偷的女士。
[whose指代 the woman’s，在从句中作定语, 修饰car。]

This is the house whose wall is painted pink. 
这就是墙壁被刷成粉红色的那幢房子。
[whose表示“房子的”，在从句中作定语, 修饰wall。]

The boy whom I spoke to is a nice person. 
与我讲话的那个男孩子是个友好的人。
[whom指代 the boy，在从句中作spoke to宾语。]

The town where she lives is very small. 
她所居住的那个小镇非常小。
[从句意指：she lives in the town]
[where意指in the town，在定语从句中作地方状语]

I can still remember the day when you were born. 
我仍然记得你出生的那天。
[从句意指：you were born on that day]
[when意指on that day，在定语从句中作时间状语]

The reason why he did it is unknown. 
他做这事的理由没人知道。
[why意指for the reason，在从句中作原因状语]

以上就是定语从句的基础解析。要查看例题，点击查看：
定语从句讲解