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"Who" is the subject; whom is the object.
When you are not certain which to use, use "he" and "him" to help you. ("He" is the subject, "him" is the object.)
Steve gave Tom a basketball.
He gave him a basketball.
Who gave whom a basketball?
Take this sentence: Sarah gave the kitten to who?
Change it to each of the following:
"Sarah gave the kitten to he?" or "Sarah gave the kitten to him?" By doing this, you can see that the second sentence is the accurate one. Since "him" is the object, you should use the object "whom." So the question should read, "Sarah gave the kitten to whom?"
Who gave you that candy bar?
1. He gave you that candy bar?
2. Him gave you that candy bar?
In this case, #1 is correct and "He" is the subject, so "Who," not "Whom," is correct.
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This might simplify it:
Use the he/him method to decide which word is correct.
he = who
him = whom
Who/Whom wrote the letter?
He wrote the letter. Therefore, who is correct.
For who/whom should I vote?
Should I vote for him? Therefore, whom is correct.
Above from Grammar Girl.