一，定语从句概念
定语从句也称之为形容词性从句，简单来说就是一个句子做定语，用一个句子来说明名词或代词（即先行词）的性质，特征等。
结构：1). 主语+谓语动词+宾语（名词或者代词）+ 关系词 + 句子。
I remember the day when we first met.
2). 主语（名词或者代词）+关系词+句子 +其他。
例句：The woman who is standing behind the counter served me yesterday.
2. 关系词：紧跟先行词后，在从句中充当一个句子成分，作宾语可以省略。
关系代词：who（主语/宾语）,whom（宾语）,whose（定语）, that （主语/宾语）, which（主语/宾语）,as （主语/宾语)- 关系代词选择：先行词为人或者物： whose, that, as 先行词为人：who, whom 先行词为物：which 只用that:
1) 先行词为不定代词（all, everything等) It is all that I know.
2) 先行词被序数词修饰 This is the first book that I bought.
3）先行词被最高级修饰 It is the best movie that I have ever seen.
4）先行词同时包含人和物 I like the book and the teacher that help me learn English.
2. 关系副词：
作状语1） when(时间状语）I still remember that day when we first met.
2）where（地点状语）This is the park where we often play balls.
3） why （原因状语）That's the reason why I didn't go to the party.
二，定语从句分类1.限制性定语从句 特征：无逗号，与主句相连，不能删除，否则句意不完整。
例句1：The woman who is standing behind the counter served me yesterday.
- 先行词 ： the woman -- 主句主语- 关系代词：who --从句主语
2.非限制性定语从句特征：与主句逗号隔开，能删除，不影响句意。关系词不可用that.
例句1：The woman ，who lives in London, will come to Shanghai.
- 先行词 ： the woman - 主句主语- 关系代词：who - 从句主语
特殊用法：1.which 可指代整个主句He forgot the umbrella , which made him get wet in the rain.
2. as 引导的非限制性定语从句位置灵活。He is very careful in his work, as his boss often says. As we all know, the Earth moves around the Sun. 三，总结：定语从句的核心是关系词的选择和从句逻辑的完整性。
先确定先行词是人，物，时间还是地点；
2. 判断关系词在从句中作主语，宾语还是状语；
3. 根据限制性或非限制性选择是否用逗号；
4. 注意特殊规则（如只能用that 的情况）
