It doesn’t concern many people actually because the pronoun whom has been steadily falling out of use over the last hundred years or so. It’s rarely used in speech nowadays and there is speculation that it will soon become extinct. But you’ll still find it in formal writing and many writers pride themselves on using it correctly.

As whom declines, who is used more and more and these two words may seem interchangeable.  But there is a difference. Who functions as a subject in a sentence whereas whom functions as an object.

As the subject of a sentence, who performs actions:

• Who rescued the drowning woman?
• I’m not sure who put the toys away.
• Do you know who left this coat behind?

Who is doing the rescuing in the first sentence and in the same way who did the putting away and the leaving behind in the other examples.

As the object of a verb or preposition, whom is the receiver of actions:

• Whom did you see?
• His grandchildren, whom he loves dearly, are in town for a visit.
• The cleaner, whom we hired last week, failed to come to work today.

In the first sentence, whom is being seen, not doing the seeing. In the other examples, whom is being loved and hired. Whom is the direct object in all three sentences.

Whom also functions as an indirect object where the person is on the receiving end of the action:

• She gave whom the package?
• Whom should I call first?
• My brother doesn’t remember to whom he e-mailed the questions.

But language develops in spite of rules and in most informal speech and writing, people will opt for who where whom has traditionally been used. This choice sounds much more natural to most native English speakers.