关系代名词代替子句内失踪的名词，作为“替身”，因此也有“格”的分别，在关系子句中当作主词就用主格who，扮演受词则用受格whom，不过代替无生物的which则没有格的区别：
I ran into a colleague [who was shopping for a cellphone].
→who代替colleague，在关系子句中为主词
I ran into a colleague [whom I truly admire _______ ].
→whom代替colleague，在关系子句中为受词
I ran into a colleague [whose car was parked next to mine].
→whose代替colleague’s，在关系子句中为“所有者”
I like any food [which tastes good].
→which代替food，在关系子句中为主词
I only like the food [which my mom made _____ ].
→which代替food，在关系子句中为受词
He told a story [that shocked everyone].
→that代替story，在关系子句中为主词
He told a story [that I heard _____ several times].
→that代替story，在关系子句中为受词
上述用法中，作主词的who已渐渐取代whom，造成who/whom不分的现象，原因可能和“懒人原则”有关。此外，当who/which作主词时，一定要出现，但是作受词时，则可以省略，因为不致造成理解困扰。
——劉美君. (2012). 英文文法有道理! 重新認識英文文法觀念. 聯經.
