Just as many people think “I” is a classier pronoun than “me,” they also think “whom” is more la-de-dah or professional than “who.” False in both cases; it just depends on whether you need a subject pronoun or an object pronoun.
Let’s look at a couple of examples:
1. Who/Whom let the dogs out? If you can substitute any of these subject pronouns—he, she, we or they—use who. We let the dogs out. She let the dogs out. You need who in this sentence.
2. Ask not for who/whom the bell tolls. That bell tolls for her/him/us/them. When an object pronoun works, use whom.