Explanation: whom is object pronoun which applies to people, which means it describes the person on the receiving end of action that was actually performed, e.g I don't know for whom he called, from here the main clause is(I don't know) and the relative clause( whom he called). The same goes for "To whom should we send the check" to whom(main clause) while "To whom should we" (relative clause), send the check(main clause)...
