1. 在 all, nobody, no one, somebody, someone, anybody 等词之后两者都可用，但以用 that 较普通。如：
All that [who] heard him were delighted. 所有听了他讲话的人都很高兴。
Have you met anybody that [who] has been to Paris? 你遇见过到过巴黎的人吗?
2. 在序数词、形容词最高级以及 the only, the same, the very 等之后两者都可用，但以用 that 较普通。如：
Newton was one of the greatest men that [who] ever lived. 牛顿是世界上最伟大的人物之一。
He is the only one among us that [who] knows Russian. 他是我们中间唯一懂俄语的人。
3. 引导非限制性定语从句时，通常只用 who。如：
Mrs Smith, who has a lot of teaching experience, will bejoining us next term. 史密斯夫人很有教学经验，她下个学期将来与我们一道工作。
4. 但是在下列情况，通常只用 that:
(1) 当先行词是一个既指人又指物的并列词组时。如：
I made a speech on the men and things that I had seen abroad. 我就我在国外所见到的人和事作了报告。
(2) 当先行词是 who 时(为避免重复)。如：
Who that has sympathy can laugh on that occasion? 有同情心的人谁会在那个时候发笑呢?
(3) 当关系代词在定语从句中作表语时(可省略)。如：
Tom is not the boy (that) he was. 汤姆这孩子已不是以前那个汤姆了。
