Learner's definition of WHO
1
a : what or which person or people谁；什么人 —
used when you do not know the name or identity of a person or group of people that you are talking about or asking about用于不确定所谈论或询问对象的名字或身份时 -
Who will be the next president?谁会是下任总统？
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Who is standing next to you in this photo?照片里站在你旁边的人是谁？
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I didn't know who he was.我不知道他是谁。
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Find out who they are.查清楚他们的身份。
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I wonder who else signed up.不知道还有谁签了字。
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Who should walk in [=who do you think walked in] when we least expected it? My parents!我们最不期待的人进来了，你猜是谁？我父母！
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She isn't sure who she talked to.她不确定跟自己说话的是谁。
b —
used to question a person's character or authority用于质疑某人的身份或权威 -
Who are they to tell us what to do? [=what makes them think they can tell us what to do?]他们凭什么指挥我们？
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Who do you think you are? [=what gives you the right to say or do this?]你以为你是谁？
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Who do you think you're talking to? [=what gives you the right to speak to me in this (rude) manner?]你知道你在跟谁说话吗？
c —
used in questions that are meant to say that no one would or would not do something, know something, etc.谁会…，谁不…，谁知道（用于疑问句） -
They may come or not. Who cares? [=no one cares; I don't care]他们可能来也可能不来。管它呢。
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Of course I would love to win a million dollars. Who wouldn't? [=anyone would]我当然想赚一百万美元。谁不想啊？
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“Are they coming?” “Who knows?” [=no one knows; I don't know]“他们会来吗？”“谁知道呢。”
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Who are we kidding? [=we aren't kidding/fooling anyone]我们可不是开玩笑。
2
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used after a noun or pronoun to show which group of people you are talking about用在名词或代词后，指代谈论到的某一类人 -
Subscribers who are interested in joining the discussion group should contact the secretary.有意参加讨论小组的订阅用户须联系秘书。
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The people who conducted the study were extremely professional.开展这项研究的人非常专业。
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Someone who handles stress well will be more suited for the job.能很好应对压力的人更适合此项工作。
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I have a friend who my parents don't like.我有个朋友，我父母都不喜欢。
3
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used to introduce an additional statement about someone who has already been mentioned用于引出对于所提及对象的补充说明 -
His patient, who was a well-known television personality, suffered a minor stroke.他的病人，一位电视名人，患了轻微中风。
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The former president of the company, who is retired now, is credited with expanding and improving the product line.生产线的拓展与改善归功于公司现已退休的前任总裁。
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Her uncle, who she's very fond of, lives next door.她非常喜欢的那个叔叔就住在她隔壁。
who's who
1
or who is who : information about the people who make up a group谁是谁（某一群体的人员信息） -
We had lived in this town long enough to know who was who. [=to know the names of the different people, what they did, etc.]我们在这个镇住了很长时间，对这里的人很熟悉。
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I'm still learning who's who around the office.我还在熟悉办公室每个同事的情况。
2
a : a list of the names of the important and well-known people in a particular field名人录 b : the important and well-known people in a particular field名人；要人