定语从句是初中到高中语法的重头戏，单选、语法填空都考关系词选择。解题只有三步：找先行词、判断先行词在从句中充当什么成分、选对应关系词。规则全部表格化。
关系代词用法表
| 关系词 | 先行词 | 在从句中充当 | 例句 |
|---|---|---|---|
| who | 人 | 主语 | The boy who is standing there is my brother. |
| whom | 人 | 宾语（可省略） | The girl (whom) I met is my cousin. |
| which | 物 | 主语或宾语 | The book which lies on the desk is mine. |
| that | 人或物 | 主语或宾语 | This is the photo that I took in Beijing. |
| whose | 人或物 | 定语 | I know the boy whose father is a doctor. |
关系副词用法表
| 关系词 | 先行词 | 例句 |
|---|---|---|
| where | 地点 | This is the school where I studied for six years. |
| when | 时间 | I still remember the day when we first met. |
| why | 原因 | I don't know the reason why she was late. |
三步判断法：第一步圈出先行词是人、物、地点、时间还是原因；第二步看从句缺主语宾语还是缺状语；第三步缺主宾选关系代词，成分齐全只缺状语选关系副词。从句中关系词作宾语时可以省略。
易错点：必须用 that 的四种情形
先行词是 everything、anything、nothing、all 等不定代词，或被序数词、最高级、the only 修饰时，关系词用 that 不用 which——All that can be done has been done。反过来，非限制性定语从句（先行词后有逗号隔开）只能用 which 或 who，不能用 that。判断方法：先看有没有逗号，有逗号排除 that；再看先行词是不是不定代词或被最高级修饰，是就锁定 that。考题在这两个方向反复设坑。