In the same way “I” and “me” are equivalent pronouns, depending on which one is needed (see post a few below), “who” and “whom” are also equivalently weighted, although people throw in “whom” when it’s not needed because they think it is more elegant or classy. It isn’t.
As “I” is a subject pronoun, so is “who.” “Me” is an object pronoun, as is “whom.” How do you know which one to use? It’s easy:
The winning run was hit by a player (who/whom) I used to know in college.
Here’s what you do:
1. Cover up the whole sentence up to and including the “who/whom.”
2. Read what is left: I used to know in college.
3. You’ll see a word is missing: I used to know HIM in college.
4. If the missing word is a subject pronoun (in this case, it would have been HE), you want the subject pronoun WHO. But in this sentence you need HIM, an object pronoun, so you use the object pronoun WHOM.
5. That’s the trick.
For your convenience, here are lists of the subject and object pronouns:
SUBJECT PRONOUNS: I, she, he, we, they, who
OBJECT PRONOUNS: me, her, him, us, them, whom
“It” and “you” can be either subject or object pronouns, so don’t use them in your test. And often, as in the sample sentence above, you can eliminate WHO or WHOM. But when you do need one of those pronouns, follow these steps and you’ll always be right.