Many of the AP Stylebook users who responded to the Grammar Day Twitter question (AP Quiz Top Two Anathemas ) complained about the usage of who and whom.
For all practical purposes, the pronoun form whom is ready to go the way of ye, an old form of the pronoun you.
Ideally, speakers who do not understand that who is a subject form and whom is an object form would simply stop using whom altogether. The forms are so similar that we can get along just fine by using who for both. Millions of English speakers already do.
Speakers who have mastered the grammar concepts transitive verb and object pronoun are free to use both forms until a generation arises that knows not whom.
That time may be delayed, however, by a segment of English speakers who do not understand the object function of whom, but who have decided that whom must be a more elegant way of saying who.
The forms who and whom function like the other pronouns, such as he and him, she and her, and I and me. The first form in each pair is used as the subject of a verb. The second form is used as the object of a verb or preposition.
Admittedly, the subject/object forms of the personal pronouns are under siege from speakers who use subject forms as objects and vice versa, as in these examples from the web:
Me and my friends are in class.
This trip proved to my husband and I that we can still travel.
CORRECT: My friends and I are in class.
CORRECT: This trip proved to my husband and me that we can still travel.
Nevertheless, I/m, he/him, she/her, we/us, they/them have not yet reached the state of confusion that exists with who/whom.
Who as the subject of a verb
Who is that masked man? (Who is the subject of the verb is.)
Garett Morgan is the man who invented the yellow traffic light. (Who is the subject of invented.)
Whom as the object of a verb
The first employee they hired was Jeff Johnson, whom Knight had met at Stanford. (Whom is the object of had met.)
Whom do you prefer in this election? (Whom is the object of do prefer.)
Whom as the object of a preposition
Figure out how much you owe, to whom and on what terms, and start paying it off. (Whom is the object of the preposition to.)
The Daniels have five children, three of whom are adopted. (Whom is the object of the preposition of.)
Incorrect uses of whom (These examples are from printed sources.)
They were aware of students participating whom had not participated in the past.
. . . a detained Palestinian whom, according to police, stabbed two people in a supermarket.
Springdale police are searching for this man whom they say robbed an Arvest Bank branch Thursday.
CORRECT: They were aware of students participating who had not participated in the past. (Who is the subject of had participated.)
CORRECT: a detained Palestinian who, according to police, stabbed two people in a supermarket . . . (Who is the subject of stabbed. Beware of intervening phrases like “according to.”)
CORRECT: Springdale police are searching for this man who they say robbed an Arvest Bank branch Thursday. (Who is the subject of robbed. “They say” intervenes between the subject and verb of a relative clause.)
The best advice for writers and speakers for whom this grammatical concept represents kryptonite usage is this: Stop using whom altogether.
If, however, you want to keep trying, here is another review: Who vs Whom</em>.