All right, people, we have been here before. I'm going to go over this one last time and then be done with it. Most of you should just give up on whom and not look back.
I have told you before: The question I have been asked most frequently during nearly forty years as a professional editor is "Should this be who or whom?" That's from college-educated professional journalists who write formal English for a living. I also notice that it keeps cropping up on discussion boards for editors. And writers who persist in using whom generally get it wrong half the time or more.
I will, however, sum up the grammar for you, so that you can make your own informed choices.
The place where whom survives most tenaciously is when it is simply the object of a preposition: To whom did you send that email? But even there it's a little dicey. Who did you send that email to? sounds less prissy and pompous. We have got to the point where using whom, even correctly, can sound pretentious.
The place where most people fall afoul of the grammar is in the use of subordinate clauses.
Here's a correct use: The job of the elections board is to certify the candidates whom the public chose. Substitute a personal pronoun for the sense: the public chose them.
Here's a common incorrect use: The job of the elections board is to certify whom got the majority of the public's votes. You'll be looking at the infinitive to certify and expecting an object; and there is one, but the object is the whole noun clause in which who should be the subject—they got the majority.
Since this grammar no longer comes naturally to many people—and probably never did to most—choosing between who and whom means painstakingly sifting the grammar of clauses to determine whether the pronoun is functioning as a subject or an object.
Or just use who.
Now there are sticklers whose empurpled wattles will quiver with rage and scorn when you use who in place of whom—God above, you can still find people in this century who insist on the default masculine and carp about terminal prepositions—but their remaining time on this side of the turf is limited. Chances are increasingly good that you do not need to cater to them.
If your text passes through my hands, I'll still make it right, but neither am I going to be on this side of the turf forever.
NOTE: Katherine Barber kindly pointed out a mistake I made in revising the entry, with the correct usage given under the incorrect label. It has been fixed, with thanks to her.