WHOM or WHO?

There's something about these two pronouns that for some reason, they are in the group of "confusable" words; so let's see if the following examples clarify a little bit ...

WHOM is a pronoun formally used instead of  WHO as the object of a verb or preposition.
Examples:
- I met a man with whom I used to work.
- He took out a photo of his son, whom he adores.
- There were 500 passengers, of whom 121 drowned.
- To whom do you wish to speak?

As I said above, "whom" is more formal than "who" and you probably see it in a formal letter "To whom it may concern ..."
Conclusion? Don't worry about using in in your every day language since it is not "common".

Hope this help. Teacher Silvia