先行词指人时，关系代词既可用who，亦可用that。但关系代词在从句之中用作主语时，多用主格who。如：

Persons who are quarrelsome are despised. 好争吵者遭轻视。(除外persons，还有people，those，等皆多用who)

All who heard the story were amazed. 听到这个故事的人都感到吃惊。(代词如he，they，any，all，one等之后多用who)

I will pardon him who is honest. 我愿意宽恕他，他是诚实的。(描述性定语从句用who)

I think it is you who should prove to me. 我认为是你应该向我提出证据。(在强调结构中多用who，who在此可省去)

Who is not for us is against us. 谁不赞成我们就是反对我们。(缩合连接代词who为可代之以that)

在下列一些情况中则多用that。如：

)He was the man that the bottle fell on. 他就是瓶子落在其身上的那个人。(此处常用that作宾语指人，亦可用whom)

He is a man that is never at a loss. 他是一个从未一筹莫展的人。(that常用于泛指人)

He was watching the children and parcels that filled the car. 我望着塞满车的孩子和包裹。(兼指人与物时须用that)

Who that you have ever seen can beat him in chess? 你曾见过谁能在棋艺上打败他?(避免与先行词who重复时应用that)

That’s the same man that asked for help the day before yesterday. 这个与前天求援的是同一个人。(先行词前有指示代词same时应用that)

He is not that man that he was. 他已不是过去的他了。(that常用作表语)

I knew her father for the simplest, hardest working man that ever drew the breath of life. 我早知她的父亲是一个世上最简朴最努力工作的人。(先行词前有形容词最高级、序数词或only等词时应用that)

希望我能帮助你解疑释惑。