who 先行词指人，在定语从句中充当主语，有时也可做宾语
whom 先行词指人，在定语从句中充当宾语。
whose 先行词指人或物，在定语从句中充当定语。
e.g. 1. The man who is talking with my father is our headmaster.
（who指the man，从句中做is talking with的主语）
2. The man whom my father is talking to is our headmaster.
（whom指the man，从句中做is talking to的宾语）
3. The man whose son is a doctor is our headmaster.
（whose指the man，从句中做son的定语，即the man's son)
