1．先行词是人，关系词在定语从句中作主语时，用who/that，关系词不可省略；关系词在定语从句中作宾语时，用whom/who/that，关系词可以省略。
2．先行词those后常用who引导定语从句。
3．“介词＋关系代词”引导的定语从句，如果先行词指人，用whom指代人且不能省略。
4．whose引导定语从句时，作定语，且不能省略。
I've become good friends with several of the students in my school who/whom/that I met in the English speech contest last year.
我与好几位去年在英语演讲比赛中遇到的同校同学成了好朋友。
I have many friends to whom I'm going to send post cards.
我要寄贺卡给我的很多朋友。
The school shop, whose customers are mainly students, is closed for the holidays.
这家主要顾客是学生的校内商店，放假时关闭。
点津：关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。
Tom is the only one of the engineers who has come up with the solution.
汤姆是这些工程师中唯一一个想出解决方法的人。
