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If you've been linked to this, you probably made a language mistake and whoever sent this to you cared enough to point it out. The best thing to do is read the details and see where you went wrong.
"Who" is used when the person being asked about is the subject of the sentence. "Who is it?" "Who did this?" "Whom" is used when the person being asked about is an object. "According to whom?"
The subject-object difference is the same when using these words as relative pronouns. "Who" is used if it refers to the subject in the relative clause. "The person who gave me this is my friend." "I gave it to the people who were there." "Whom" is used if it refers to an object in the relative clause. "She went to the one whom she trusted most." "The person whom she told listened."
Many people don't care if you use "who" when "whom" is needed, but please don't use "whom" if "who" is correct.
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