You use who after certain words, especially verbs and adjectives, to introduce a clause where you talk about the identity of a person or a group of people.
Police have not been able to find out who was responsible for the forgeries.
警察还未能发现谁是那些伪造案的元凶。
I went over to start up a conversation, asking her who she knew at the party.
我走过去和她攀谈,问她在聚会上都认识谁。
You use who at the beginning of a relative clause when specifying the person or group of people you are talking about or when giving more information about them.
There are those who eat out for a special occasion, or treat themselves.
有些人会在特殊的日子或犒劳自己时出去吃饭。
You use who in questions when you ask about the name or identity of a person or group of people.
Who's there?
那是谁?
Who is the least popular man around here?
谁是这里最不受欢迎的人?
'You reminded me of somebody.'—'Who?'
“你让我想起了一个人。” “谁?”