What are we to do with a sentence like this: You were sitting next to the person whom everybody thinks will be the next president of the corporation. Or is it who everybody thinks? It’s a perfect example of how practically important it can be to train your eye to see clauses first when grammatical questions arise.
The sentence consists of three clauses (three pairs of subjects and predicates): you were sitting, whom (or who?) will be, and everybody thinks. Grammatical analysis always proceeds clause by clause, because every clause, whether independent or subordinate, holds to its own laws; every clause, that is, must be grammatically sound in and of itself, standing in relation to other clauses, of course, but comprising a syntactical integrity in its own right. If we can be patient, then, to work methodically through a longer sentence like this one, remembering that it is common for us to interrupt ourselves when we’re writing or speaking, we’ll find ourselves ready to assemble the pieces when we reach the period.
The main clause of the sentence is the first one, you were sitting next to the person. No trouble here. A grammatically complete thought, to be sure, but one which logically needs to immediately abut a relative clause to make the reference to person more specific: the person whom (or who?) will be the next president. Note that there are, correctly, no commas around the relative clause because it is working restrictively to define the word person. (Only nonrestrictive relative clauses are marked by commas.)
Now it is right here that we come upon the dragon in his lair. If it is true that every clause must hold its own syntactical integrity, then we should keep in view only this relative clause for a moment as we analyze it—even though everybody thinks, the third clause, interrupts it: whom (or who?) will be the next president. The verb of this relative clause, will be, needs a subject, and subjects are in the nominative case. Who is the nominative form of the relative pronoun (whom is the objective case), and so in pretty quick order we can confidently say that the sentence should read You were sitting next to the person who everybody thinks will be the next president of the corporation.
But the dragon stirs: No, no, thinks is a transitive verb, and transitive verbs require an object, so it should be whom everybody thinks. But however easily our ear may register those three words as a unit, grammatically and logically they do not belong together. What it is that everybody thinks is not just whom, meaning the person, but the complete idea, the person who will be the next president. The object of the transitive verb thinks, in other words, is the entire relative clause, not simply the relative pronoun alone. Grammatical coils can be tricky sometimes to slip.
So again, the sentence should read You were sitting next to the person who everybody thinks will be the next president of the corporation. Doubt about the correct form of the relative pronoun arises when we rely too soon on our ear, on the way something sounds. Once trained, our ear plays an essential rhetorical role in designing sentences to conform to the meaning we intend, but if it is sent out too early to decide matters of grammatical and logical fact, its enthusiasm can play havoc with the more sober procedure of analysis. Grammatical facts are the foundation and rhetoric the flourish, and the art of composition, like the art of building, requires the foundation first.