The word whom functions as pronoun (interrogative or relative) in sentences. It’s the objective form of pronoun who and hence is used as object of verb or preposition.
Whom as interrogative pronoun
Whom can function as an interrogative pronoun. An interrogative pronoun (who, whom, whose, what, and which) is a pronoun used to ask direct or indirect questions. Examples:
(Comments are in square brackets.)
Whom should I contact for more information?
At whom did he shout? [It can also be written as Whom did he shout at?]
I don’t know whom I should contact for more information. [Indirect question. It’s also known as wh-noun clause]
Do you know whom he shouted at? [Indirect question]
Some mistakenly treat whom introducing a noun clause as subordinating conjunction. That’s because, in common parlance, the term subordinating conjunction is associated with any word that joins a dependent clause to an independent clause. But in noun clauses, none of wh-elements (how, when, where, why, who, whom, whose, what, which) is a subordinating conjunction. They are either interrogative adverb, interrogative pronoun (whom, for example), or interrogative determiner.
Whom as relative pronoun
Whom can function as relative pronoun as well. A relative pronoun, if you recall, introduces a relative clause, which is placed immediately after the noun phrase it modifies. Examples:
The author, whom you mentioned few weeks back, is signing books at the local library. [Relative clause has been underlined]
The judge, to whom the case was assigned, is retiring next week.
Some mistakenly treat whom introducing relative clauses as subordinating conjunction. This error though is less common than the one in noun clause. But whom, in this role, is a pronoun – relative pronoun – and not conjunction.
If you noticed, whom introduces two dependent clauses – relative and noun clause. Since it doesn’t introduce adverb clause, it’s not a subordinating conjunction.
