John Knight, who/whom I spoke to yesterday, seemed to be rather irritable.
In this sentence, John Knight is an object because I (the subject) am speaking to him; however, he is also a subject since he is performing an action, which in this case, is being irritable. Is he both a subject and an object? If not, which one of these is he?
Furthermore, which is correct: who or whom?
The same word cannot be doing two different jobs at once.
John Knight, who/whom I spoke to yesterday, seemed to be rather irritable.
Your thesis that John Knight is “both” a subject and an object here is where you’ve gone wrong. Your syntactic analysis is off, in that it isn’t ever an object here. It is only the subject, the subject of the verb seem.
In contrast, the subject of your subordinate clause is I and the object is the relative pronoun who/whom.
Which provides you your answer: it should “really” be whom in careful speech and writing, since it’s the object not the subject. This “really” bit is that this is a “rule” that not all native speakers and writers are aware of — or even if they are aware of, choose to follow.
