what is what, which is which与who is who
这三个表达在口语中非常常用，由于其结构相同，用法相似，所以经常被弄混。本文拟对其用法区别作一归纳。
一、what is what
what’s what在口语中通常与动词know, learn, show, tell, perceive等连用，其主要用法有：
1.表示“哪个是哪个”。如：
The weeds and the flowers are coming up together, and we can’t tell what is what. 杂草和花长在一起，我们都没法区别哪是草哪是花。
It will take you a few weeks to learn what’s what in the job. 你要花几个星期的工夫才能熟悉这项工作。
2.表示“好东西与坏东西”“有用的东西与无用的东西”“青红皂白”。如：
He can’t tell what’s what. 他好丑不分(from www.lulutubu.com)。
You’re old enough now to know what’s what. 你已经不小了，应该知道好歹了。
3.表示“什么事情有用”“什么事情重要”。如：
She certainly knows what’s what. 她当然知道轻重缓急。
4.表示“事情的真相”“事情的究竟”。如：
I keep my ears open and I know what’s what around the office. 我一直在注意听，知道办公室发生的情况。
If he gets angry enough, he’ll tell you what’s what. 假如他生气到一定的程度，他会告诉你到底是怎么一回事。
5.表示“事情的诀窍”。如(from www.lulutubu.com)：
When it comes to cooking, Jenny knows what’s what. 谈到烹饪，詹妮是内行。
Linda’s been in the business for thirty years—she knows what’s what. 琳达在这一行当里干了30年，很老练了。
二、which is which
which is which的意思“哪个是哪个”，通常用于区分两个十分相似的人或事物，一般与动词tell, know等连用。如：
You don’t know which is which, do you? 你不知道哪个是哪个，对吗？
These kittens look exactly alike—how can you tell which is which? 这些小猫看上去一模一样，你怎么能分出哪个是哪个?
The twins are so much alike that I can’t tell which is which. 这对双胞胎长得可真像，我都分不出谁是谁了。
Tag the bottles now or we’ll forget which is which. 把这些瓶子加上标签，否则的话，我们就忘了哪个是哪个了。
注：有时可用which…is which这样的形式。如(from www.lulutubu.com)：
It took three months before we knew which end of the ship was which. 我们花了三个月时间才弄清楚哪是船头哪是船尾。