The difference between who and whom
TLDR: Who takes the subject position in a sentence, whereas whom takes the object position, usually.
Despite the fact that the word whom is beginning to decline in the English language, it is still commonly used in formal writing… and by Stephen Fry and Terry Pratchett, who I love!
who (pronoun)—the subject case—which person or people; used as an introductory device to a secondary clause relating to a person or people previously mentioned
‘Who did this?’ the teacher demanded.
‘Does anybody know who was behind the murder?’ Watson asked.
‘This woman here is the actress who is going to play the heroine.’
My cousin, who has come to stay with us for a week, is very stuck up.
whom (pronoun)—the object case—used as the object of a verb or preposition
Our veteran sergeant, in whom we all trusted, instructed us to prepare for war.
‘To whom does this sock belong?’ the nanny asked, holding her nose.
To whom it may concern, I am writing this letter with the utmost disappointment.
‘Whom did Jonathan run away with?’
Since these two words trip us up quite often, there is a fairly easy system to adopt to ensure you’ve used the right one in some instances of its usage. Just like who and whom, she and her adhere to the subject/object case rules. Try to reword your sentence (or answer the question), replacing who with she and whom with her. For example:
‘Who did this?’
becomes
‘She did this.’
She sounds fine, whereas her does not. Therefore who is correct.
‘Whom did Jonathan run away with?’
becomes
‘Jonathan ran away with her.’
Her is grammatically correct, meaning whom is also correct in this sentence.
However, in spoken English—and quite often in written English now, come to think of it—who is replacing whom with a slight rewording of the sentence. For instance:
‘To whom does this sock belong?’ the nanny asked, holding her nose.
has become
‘Who does this sock belong to?’ the nanny asked, holding her nose.
The second sentence is not wrong, per se, it just means you end your question with a preposition, which many writers strive very hard to avoid.
Though whom is increasingly being replaced with who, whom is not yet dead and still holds a valuable place in the English language. However, it is also important to note that the use of who is not wrong in our ever-changing language, it is simply different and will probably, one day, be the predominant way of expression.