According to the rules of formal grammar, who should be used in the subject position in a sentence, while whom should be used in the object position, and also after a preposition.
Subject and object
What we’re essentially looking at is the difference between the subject and object in a sentence.
Who is used for the subject (equivalent to she,we, he or they);
whom for the object (like her, him ,us or them)
The old rule was straightforward: Use who when the pronoun is a subject, as in "Who wrote this nonsense?"; use whom when the pronoun is an object, as in "To whom should I send this nonsense?"
See if you can choose the correct usage in the following sentences:
1. "Who/whom did this?"
2. "Who/whom should I turn to?"
3. "The person who/whom wrote this report did an excellent job."
4. "I don’t know who/whom to trust."
5. "I know who/whom is responsible for this mess."
In the first sentence who is the subject. In the second sentence whom is the object of the preposition to. (In formal or traditional usage, that sentence would be reordered: "To whom should I turn?") In the third sentence, who is the subject of the subordinate clause "who wrote this report." (The main clause is "The person . . . did an excellent job.") In the fourth sentence whom is the object of the infinitive to trust.
The fifth sentence is tricky. At first you might think whom is the object of the verb know, but look closely. Who serves as the subject of the subordinate clause "who is responsible for this mess." In sentences like this one, pronoun case (the subjective case who or the objective case whom) is determined by how the pronoun functions in its own clause.
So, should it be who or whom in the following sentences?
1. "I wasn’t sure who/whom to vote for."
2. "She knew who/whom would do a better job.
3. "Tell me who/whom won the race."
If you chose, whom, who, and who, you know the rule. Now for the challenging part. (Did you think we were already there?)
The rule is changing. Perhaps because whom sounds formal and stilted to the modern ear, it is becoming less common. Who is becoming acceptable where whomwas called for by traditional grammar, as in "Who can I turn to?"
I’m fine with that. I’m no purist. When language changes to reflect the spirit or ethos of the times, I’m all for it – as long as the change doesn’t diminish our ability to communicate complex ideas precisely, colorfully, and artfully.