定语从句在句中作定语,用来修饰某个名词或代词,而定语从句位于先行词的后面。
关系词分关系代词(that, which, who, whom, whose, as)和关系副词(when, where, why)。
关系词引导定语从句,指代先行词,并在从句中作一定成分,如作主语、宾语或状语等。关系代词用来引导定语从句.

关系代词who / that的区别:that可指人或物,而who只指人
He is a person that does what he says. (person是先行词, that在从句中作主语)他是一个说话算数的人。
The cat that l bought yesterday is ill. (cat是先行词,that在从句中作宾语)我昨天买的那只猫病了。
A friend who helps you when you have trouble is a real friend.(friend是先行词,who在从句中作主语)
在你有困难时帮助你的朋友才是真正的朋友。