定语从句像一条"小尾巴"，跟在名词后面修饰它，说明"哪一个/什么样的"。
引导词的选择
先行词是人：who / that
The girl who is singing is my classmate.（正在唱歌的那个女孩是我同学。）
先行词是物：which / that
I like books which/that have interesting stories.
关系词在从句中作宾语时可省略：
The film (that) we watched yesterday was moving.
只能用 that 的情况（常考）
先行词是 everything, nothing, something, all 等不定代词
先行词被序数词或最高级修饰：the first..., the best...
This is the most interesting book that I have ever read.
主谓一致
从句动词与"先行词"保持一致：
I have a friend who likes drawing.（先行词 a friend 单数 → likes）
I have many friends who like drawing.（先行词 friends 复数 → like）