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）