"Who" Or "Whom"?
There’s an ongoing debate in English about when you should use who and when to use whom. 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.
Who made this decision? [here, who is the subject of the sentence]
Whom do you think we should support? [here, whom is the object of support]
To whom do you wish to speak? [here, whom is following the preposition to]
In practice, most people never use whom like this in speech because it sounds extremely formal. They don't use whom at all, and instead use who in all contexts, i.e.:
Who do you think we should support?
Who do you wish to speak to?
As relative pronouns
Who and whom are also relative pronouns, used to link one clause to another. For example:
The man whom you met yesterday is coming to dinner.
The children, who had been as good as gold, then suddenly started misbehaving.
The people to whom the funds were supposedly directed benefited little from them.
As with questions, using whom in the above examples is markedly formal. Most people would say or write the first example in one of these ways:
The man who you met yesterday is coming to dinner.
The man that you met yesterday is coming to dinner.
The man you met yesterday is coming to dinner.
They would also probably move the preposition to the end of the clause in the third example:
The people the funds were supposedly directed to benefited little.
However, if you are writing at work, at college or university, or for publication, it is acceptable and even advisable to use the more formal whom, especially in constructions with a preposition.
In one specific context whom seems obligatory: when it is preceded by quantifiers such as all of, both of, few of, many of, several of, etc. For example:
The Millennium Stadium accommodates 72,500 spectators, all of whom are seated.
Congratulations to all the winners, most of whom are definitely reading this blog!
Avoid this mistake with whom
In reading, you may well come across sentences like these:
✗ He is demanding £5,000 from the elderly woman whom he claims has ruined his life.
✗ Mr Reynolds is highly critical of journalists, whom he says just use labels to describe him.
Who is correct in both examples. The mistake occurs because the reporting verbs claim and say have been inserted between the grammatical subjects [elderly woman, journalists] and the verbs they govern [has ruined, use]. If you take them out, you can see that the sentence is wrong, because whom is being incorrectly used as the subject:
✗ He is demanding £5,000 from the elderly woman whom has ruined his life.
✗ Mr Reynolds is highly critical of journalists, whom just use labels to describe him.
If you find yourself writing a sentence of that kind, which can happen with verbs such as say, report, think, believe, etc., use the test above.
You can read more about the rules and guidelines about when to use who and when to use whom on the Oxford Dictionaries blog.