When are who and whom used?

To answer that question, review the following five rules:

1. Use who or whoever as the subject of a verb,
   Example:
   Who won the prize?
    
2. Use who or whoever as the subject of a clause (even if the whole clause functions as the object of a verb or a preposition).
   Examples:
   The new manager, who moved here from Texas, called a meeting for this afternoon. (Who is the subject of the verb moved in the clause who moved here from Texas.)
   
   He left notes for whoever needed them. (The entire clause whoever needed them is the object of the preposition for. Whoever is the subject of the verb needed.)
    
3. Use who or whoever after forms of the verb to be (for example, is, are, were, have been).
   Example:
   It is who? (Who is it?)
    
4. Use whom or whomever as the object of a verb.
   Example:
   Whom did you pay? (You did pay whom? Whom is the object of the verb did pay.)
    
5. Use whom or whomever as the object of a preposition.
   Examples:
   With whom were you working? (You were working with whom? Whom is the object of the preposition with.)
   
   He was the man in whom we put great trust. (We put great trust in whom? Whom is the object of the preposition in.)

GRAMMAR TIP: The easiest way to check the use of who and whom is to arrange the elements of the sentence or clause (separated from the rest of the sentence) in subject-verb-object order. Then replace who with he and whom with him. This will usually allow you to see whether you are dealing with a subject or object form and whether you have used who and whom correctly.