>_Tempest_, Act III, Scene III.
>Prospero: ...  My high charms work
>       And these mine enemies are all knit up
>       In their distractions; they now are in my power;
>       And in these fits I leave them, while I visit
>       Young Ferdinand, whom they suppose is drown'd,
>       And his and mine loved darling.

Note that Will also used "mine" where we would use "my", and he also
allowed himself to modify the same noun "enemies" with *both* a
demonstrative and a possessive  (*Have you seen this my book yet?).
Early Modern English is a bit different from Modern English.

But even by that time the use of "whom" was getting problematical. By
now it's hopeless.  Most English speakers, if they use it at all, use it
to mark the style as being hopefully formal.

If you consider the two clauses (main and relative) to *really* be
clauses, each with a subject, verb, and (potentially) objects or other
complements, the rule is simply to use the case appropriate in the
relative clause and ignore the main clause.

Exempli gratia:

 1) Both objective, an easy case
   He is afraid of the man whom I saw.
   (He is afraid of the man) + (I saw the man).
                    ^^^^^^^           ^^^^^^^
                    Objective         Objective

    Therefore use "whom" - it's correct for both.
    (Though only the use in the relative is important).

    The rule also applies in the other easy case, Both nominative:
   The man who left early is my brother
    (The man left early) + (The man is my brother)
     ^^^^^^^                ^^^^^^^
     Nominative             Nominative

 2) Objective upstairs, nominative downstairs
   He is afraid of the man who saw me.
   (He is afraid of the man) + (The man saw me).
                    ^^^^^^^     ^^^^^^^
                    Objective   Nominative

    Here's where the rule comes in.  Relative pronouns are in
    their relative clauses, and their case assignment refers to
    their role in the relative clause, not in the main clause.
    Therefore use "who", since it's the subject downstairs.

 3) Nominative upstairs, objective downstairs
   The man whom I saw is afraid of me.
   (The man is afraid of me) + (I saw the man)
    ^^^^^^^                           ^^^^^^^
    Nom.                              Obj.

    Here again, it's the relevant case in the downstairs
    clause that governs; use "whom".

And that's really all there is to it.  You're quite right, it's simple.
But it requires one to be able to keep clauses separate, and that's
something that's vanishing in spoken English.  Most speech works by
fixed phrases instead of syntactic parsing in English, an ultimate
consequence of our loss of inflectional diversity.

Bring back the Dative Case!

 -John Lawler                     More grammar
  Linguistics Program   University of Michigan
 "..and, who knows? Maybe the horse will sing."