定语从句 = 给前面的名词贴一张更长的身份证。
who：身份证修饰人，后面缺“谁做动作”。The teacher who helped me was brilliant.
which：身份证修饰物/地方。The book which she recommended was very useful.
whose：身份证里缺“谁的”。A student whose attitude is positive learns faster.
口诀：修饰谁 → 缺什么 → 怎么合

先找被修饰名词，再看身份证里缺什么：缺人做动作用 who，缺物用 which，缺“谁的”用 whose。

修饰人 → who · 修饰物/地方 → which · 表示“谁的” → whose
关键：先找被修饰名词，再看身份证里缺主语、宾语还是所属关系。