指人可用that who, 以下情况多用who;
Those people做先行，There be的结构中；
例句：
1) Those who are for the plan, please raise your hands.
赞成计划的人请举手。
2) People who create computer viruses are called hackers.
制造电脑病毒的人被称为黑客。
3) There was once a little boy who came to the net bar to play games every night.
曾经有一个小男孩每天晚上都来网吧玩游戏。
先行指人不定代，从中做主who要在；
先行词是指人的不定代词，如 one 、someone 、anyone 、nobody 等词，或指人的关系代词在定语从句中做主语，要用who, 而不用that。
例句：
1）Nobody who knows a little scientific knowledge will believe you.
懂得一点科学知识的.人都不会相信你的话。
解析：nobody是指人的不定代词，用who, 不用that。
2）The thief who pretended to be a manager was caught stealing a car.
那个装扮成经理的贼在偷车时被当场抓获。
解析：who指人，在定语从句中做主语。
两个定从一起来，不要重复你有才；
一个先行词有两个定语从句时，关系代词要避免重复使用。
A plant that can cure diseases, which was known by the ancient Chinese , is called Chinese herbal medicine. 一种能够治病的，被古代中国人所熟知的植物，叫做中药。
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