if i am correct, "who" refers to the subject of a sentence and "whom" refers to the object. so if your question is answered by correctly identifying the subject than "who" is the way to go and if your question is answered by an object than "whom" is the best bet.

so in the sentence: "Jimmy threw the ball to Billy", i would ask "who threw the ball to billy?" and "to whom did jimmy throw the ball?"

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