Who or Whom?

Daniel writes:
There is a sign at my work which reads, "If you are not sure whom somebody is, please do not answer the door." I know instinctually that it should be 'who' instead of 'whom,' but I'm not sure why.

Since 'you' is the subject, and the unknown person is the object, why is it not whom?

You're right—it should be "who." Millions of people have trouble with this one, and it's because the "unknown person" is not the object—that person is the complement.

The verb "to be" describes the subject, but it doesn't transfer an action to anything. So a noun, pronoun, or adjective after a form of the verb "to be" is the complement of the subject:
Daniel is curious.
That man must be Daniel.
It was he who asked the question. (Notice--it's not "him" because "him" would require a transitive verb.)
I don't know who he is.
And that last sentence is the equivalent of "If you don't know who somebody is." We have a subordinate clause, "who somebody is," and since the verb is a form of "to be," the pronoun will have to be "who."

"Whom" is the objective-case version of "who," just as "him/her" is the objective-case version of "he/she." We use "whom" when the person is receiving some kind of action, or is the object of a preposition:

Whom did you select for the job? (=You selected him for the job.)
From whom did this message come? (= This message came from her.)

Yes, it's confusing. I tell my students that in a few more generations, "whom" will be obsolete and we'll use "who" for everything...just as we now use "you" in both subjective and objective cases, instead of "ye" for the objective case.

Comments

[Brett]

So under this argument, you would say:
"The person in this picture is I."
"That's supposed to be we."
"Look! It's he."

Really?

BTW, 'ye' was used originally only as a plural pronoun of the second person in the *subjective* (not objective) case.

[Crawford Kilian]

Yes, really, and I know it sounds odd because almost everyone would say "me," "us," and "him." I use those constructions myself in most cases, rather than draw puzzled stares.

My dictionary tells me "ye" was also used as an objective pronoun. A good etymological search would probably clear up the question.

[Brett]

I'm not disagreeing with the idea that the sentence is wrong, that 'whom' is marked for objective case, or even that the slot in this sentence is a complement--all correct for the case cited.

It is your parenthetical, "It was he who asked the question. (Notice--it's not "him" because "him" would require a transitive verb.)" that I see as problematic.

The claim that objective-case pronouns are legal only after transitive verbs (leaving aside prepositions) is rather odd. If, as you admit, almost everyone uses the subjective case, then what makes the objective case correct? The Cambridge Grammar of the English Language (p. 459) clearly says both are acceptable, noting that "the nominative (what we're calling objective) is restricted to formal or very formal style."

As for 'ye', indeed, it was later used as both objective and subjective pronoun, the way we now use 'you'. But your example makes no sense in light of such usage.

[Brett]

I meant to write, "The claim that *other* objective-case pronouns are legal only after transitive verbs (leaving aside prepositions) is rather odd."

Apologies!

[michael murphy]

Let's not foget that subjects of infinitives are in the objective case. (I want HIM to be president.)