今天我们一起来学习下，定语从句，什么是定语从句呢？在一个复合句中修饰名词或代词的句子，叫做定语从句。
比如说：
The man who is wearing a blue and red clothes.（who之后的句子修饰前面的man，并对此进行了解释和说明）
接着，我们一起分析下句子的结构，主句+定语从句，定语从句所需要的修饰主句中紧密相连的名词或代词，我们称之为先行词，像who、which、that等词我们称之为关系词，关系词分为关系代词：who whom whose which that和关系副词：where when why。
首先，我们来看下关系代词在定语从句中的成分。
通常who，whom，whose， that可以指人，which，whose，that指物。关系代词先行词在从句中作主语、宾语或定语。强调下指人，主语只用who和that,宾语优先用whom，that作宾语可省略；指物的情况，主语只用which和that，宾语that可省略；指人又只物的情况下，主语宾语都用that。
关系副词若在从句中作状语，如果遇到时间用when，遇到地点用where，遇到原因用why。
以下是只用that的情况：
① 序数词或形容词最高级修饰先行词时
Amry is the tallest girl that I have ever met.
②先行词被every, some, no, all, any, little, much等修饰时
I’ve read all the e-mails that you send me.
③everything, something, nothing, all, anything, little, much等不定代词作先行词时
Everything that we saw in this video was not real.
④ 先行词被the only, the very, the same, the last修饰时。
This is the only one that belongs to her.
⑤ 主句已有who或which时
Who is the man that is singing next the door?
⑥ 当先行词同时指人和物时
I’ve never heard of the people and places that you talked about just now.
还有一些情况，我们需要把关系代词和关系副词做一下替换，when=on(in,during…)which where=on(in/on/at/to/in front of…)which why=for which
