Can “whom” serve complement in a relative clause?
Does the sentence "He is no longer the person whom he was ten years ago" sound natural?
Thanks in advance!
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Whom cannot serve as complement of BE in a relative clause (or anywhere else).
Whom is an objective-case form and can be used only when it stands as the object (direct or indirect) of a verb or preposition.
Thus, you may say
He is no longer the person whom I knew ten years ago.
because whom is he direct object of knew. But in your sentence, the wh- form is a subject-oriented complement of was, and must take subject case: who.
What may be confusing you here is that in colloquial Present-Day English (and mostly even in written PDE), personal pronouns acting as subject-oriented complements take the objective form: That was me ten years ago. But that is a peculiarity of personal pronouns and does not extend to relatives. In fact, the opposite tendency is found with relatives: the objective form is well on its way to extinction.
My recommendation is that in bound relative clauses† where who is the complement of BE you omit it altogether. This is acceptable in all registers (∅ marks the point at which the relativizer is omitted):
He is no longer the person ∅ he was ten years ago.
The relativizer may also be omitted whenever the who form acts as object of the verb or of a preposition left ‘stranded’ after the verb:
He is no longer the person ∅ I knew ten years ago.
He is no longer the person ∅ I played with ten years ago.
That will leave you only two cases where you have to worry about which form to use:
In clauses where the relativizer acts as subject, use who‡:
He is no longer the person who led us to victory ten years ago.
In clauses where the relativizer is object of an immediately preceding preposition, use whom:
He is no longer the person with whom I played ten years ago.
† Note, however, that in free relative clauses, where the clause acts as an NP rather than a modifier, the wh- form cannot be deleted.
We do not yet know who did this.
The conventions in these cases can be tricky; if you want to know more about this I suggest raising it as a separate question.
‡ When the relativizer acts as subject of a verb construction headed by BE it is often possible to omit both the the relativizer and the BE form. This is another tricky rule; it has been treated on ELU here. Again, if you would like to know more I suggest raising a separate question.