Who vs Whom
The basic idea is that who is the subject form and whom is the object form.
If we’re talking about the doer, we use who (ex. “The Spy Who Loved Me” because the spy is doing the loving).
If we’re talking about the “doee”, we use whom (ex. “The Spy Whom I Loved” because the spy isn’t the one loving, the spy is the one being loved).
But it’s more complicated than that. Modern standard American english is allowing for cases where who is used as the object too.
As grammarians, we have to understand that language is a one-way street.
Whom is on its way out
The only thing we need to know is that because whom is becoming archaic, it cannot be used in subject form, only in object form. Who, however can be used for both subject form and object form.
Who would’ve thought? 😉