To Whom It May Concern: who is a subject and whom is an object. Who acts and whom receives. Say what? Who is like "he" or "she" and whom is like "him" or "her." Who is collecting money for homeless kittens? He is! Then to whom does the money go? Send the money to him.
Who often begins a question: "Who do you think you are?" Unlike whom, who can be a subject, as in "Who went to dinner?" The problem with who or whom usually lies with whom, so let's look at a few examples:
"Who Are You" (album by The Who, 1978)
"Who was that masked man?" (Lone Ranger, 1930s TV show)
Whom is the confusing one. Like many objects, it often comes after a preposition (to, by, from, over), as in that start to a business letter: To Whom It May Concern. Also, "one of whom" is a popular phrase. Whom is like an ascot — kind of old fashioned but some people still rock it. Sticklers and Southerners tend to use whom, and it's lovely when used correctly. It's formal, so that's why it shows up in business letters. So pull up your Colonial breeches and look at these examples:
"For whom the bell tolls; it tolls for thee." (from a poem by John Donne, 1624)
"For Whom the Bell Tolls" (novel by Ernest Hemmingway, 1940)
"For Whom the Cowbell Tolls" (Mississippi State Bulldog online community, 2015)
It's downhill for whom! The bottom line is that if you can replace the who with "he" and it sounds right, then keep it. Who cares? He cares! If "him" works, go with whom. Whom did she marry? She married him. Since him sounds better, stick to whom, stickler.