Question

在定语从句中只用who不用that的情况。

Answer 1

1) 在疑问句中，如果开头的疑问代词是who，为了避免重复，定语从句宜使用that引导而不用who, 如:
Who is the hero that must thus be sacrificed? 谁是需要作出牺牲的英雄呢？
Who is the scientist that first discovered radium? 首先发现镭的科学家是谁？
2) 当先行词为疑问代词who, which时，如:
We all realized who that remark was aimed at. 我们都知道那话是对谁而言的。
Which of us that knows something about physics does not know this? 我们中但凡知道点物理知识的人哪一个不懂这个？
3) 当关系代词在定语从句中作表语或宾语补足语时，定语从句也不用who引导，而用that, 或者省略关系代词。
He is no longer the man that he was. 他已经不是过去的他了。(先行词在定语从句中作表语）
He is not the fool (that) you thought him. 他不是你想像中的那个傻瓜。 (注：这条句子中 the fool 在定语从句中作宾语补足语，即 thought him the fool)
4) 当先行词同时包含人和物时
The bike and its rider that had run over an old man were taken to the police station. 撞倒老人的车和事主已被带到警局。