EX1: They are interesting books.
[定语interesting: 说明 books是有趣的]
EX2: I have a blue bag.
EX3:I have something important to tell you.
EX4: Jack is reading the book that I gave him.
[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 ]
Jack is reading the book that I gave him.
[ 先行词: the book ]
[ 定语从句:that I gave him]
The man who called yesterday is here now.
[ 先行词: The man ]
[ 定语从句: who called yesterday]
Do you know the boy that is standing over there?
Have you seen the movie that we talked about?
I saw the letter which came yesterday.
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]
Do you know the boy that/who is standing over there?
[从句意指: the boy is standing over there]
I saw the letter which/that came yesterday.
[从句意指: the letter came yesterday]
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]
The girl that/who I just met was very friendly.
[从句意指: I just met the girl]
She is the woman whose car was stolen.
[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.
The boy whom I spoke to is a nice person.
[whom指代 the boy，在从句中作spoke to宾语。]
The town where she lives is very small.
[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.
[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.
[why意指for the reason，在从句中作原因状语]