在用作定语修饰其后接名词时,只用which和what,不能用who.which和what的区别是：前者用于选择范围较小或较明确的场合,后者用于选择范围较大或不明确的场合.如：
Which color do you like,black or white?黑色和白色,你喜欢哪种?
Which is bigger,the sun or the earth?太阳和地球哪个大?
What color is your car?你的汽车是什么颜色的?
但是,若指人,即使选择的范围不明确,也可用which.如：
Which [What] writers do you like?你喜欢哪些作家?
当其后不接名词时,三者均可用,which和what的用法区别依然是：前者用于选择范围较小或较明确的场合,后者用于选择范围较大或不明确的场合.如：
Which is in the box,the red one or the black one?哪一个在盒子里,是红色的那个还是黑色的那一个?
What is in the box?盒子里装的是什么东西?
至于who,它一般只用来指人,不管选择范围大还是小、明确还是不明确均可用.如：
Who won –– Tom or Mike?谁赢了,是汤姆还是迈克?
Who is your favorite poet?你最喜欢的诗人是谁?
注：当选择范围比较明确且用作宾语时,who也可以用which或which one代之.如：
Who [Which / Which one] do you like better,your father or your mother?你更喜欢谁,父亲还是母亲?