先看 that 和 who 的区别。比如“His father，that is in Hongkong，is coming home next week.” 这就错了，得用 who。that 具有限定性，前面不能随意加逗号，像个严守规则的卫士。而 who 前加逗号就成了补充说明的角色。“His father，who is in Hongkong，is coming home next week.” 此句只是介绍父亲在香港，并非从多个父亲里特指。若用 that，就好似他有很多父亲似的，除非真有这种情况。再如“I lived with Mr. Chang，who taught me English.” who 补充了张先生教英语这一信息。
接着是 that 和 which 的不同。“Last night，I received his letter，that I have forgotten to bring here.” 这是错误的，应把 that 换成 which。which 前加逗号就如同贴心导游，用来详细说明事物。而 that 主要用于限定范围。该句中“letter”后有逗号，是在说明信的情况，并非限定，所以用 which。
要记住，which 通常指物，不能指人。“He is a man which can be trusted at all times.” 显然不对，要改为“He is a man who can be trusted at all times.” 不过古英语里 which 有特殊用法，如《圣经》中的“Our Father which art in heaven.” 但现代英语不能乱用。
询问几个人中的哪一个时，要用 which of。“Who of the students asked the teacher？” 错误，正确的是“Which of the students asked the teacher？” 例如“Which of them is your father？”
指职业时要用 which。“His friend had become a lawyer，whom he wanted to be，too.” 应改为“His friend had become a lawyer，which he wanted to be，too.” 像“ They elected him president，which he had long wanted to be.”
对于 all，指人时后面用 who 或 whom。“All that we met in the street were soldiers.” 错误，应是“All whom we met in the street were soldiers.” 指物时则可用 that，如“All that I can say is this.”
最后再给些小诀窍。先行词有最高级形容词或序数词时，如“He is the greatest man that I ever saw.” 用 that；有 the only，the same，the very 时，像“He is the same person that we met at the station last night.” 用 that；是不定代词 all，any，little，much 等时，“All that live must die.” 用 that；It is - that 结构中，“It is a long lane that has no turning.” 以及先行词含人和动物时，“The train ran over a boy and his dog that were just crossing the track.” 常用 that。
关系代词虽有点复杂，但只要用心，就能轻松驾驭。