DR. ENGLISH: Who, Whom
We’ve written about the difference between “which” and “that”; how “which” tends to be more specific than “that,” but “that” usually works well and is easy. But what about the difference between “that” and “who”?
In fact, the two pronouns are not interchangeable at all, given that “who” refers to people and “that” refers to everything else (except for those folks who like to invest humanity in their pets, like Boston Terriers).
It seems to me that people would rather say “The lying, hired-gun expert that angry geoprofessionals virtually beat to a pulp” rather than “The lying, hired-gun expert whom angry geoprofessionals virtually beat to a pulp” because “that” doesn’t impose the quandary, “Is it ‘who’ or ‘whom’?”
The fact is, though, that “who” and “whom” are for people, and “that” isn’t. And as for “who vs. whom,” bear in mind that “whom” is merely the objective form of “who,” to be used as the object of a transitive verb or the object of a preposition.