Does the trick of replacing “who/whom” with “he/him” always work?

Question

In many grammar books, whenever Who vs Whom comes up, you get the rule (who is used for to refer to the subject, whom is for the object or preposition) and then you get this supposedly handy tip:

If you are confused about using who/whom, try substituting he/him or they/them to see which makes sense.
If he/they makes sense, use who.
If him/them makes sense, use whom.

e.g. [Who/whom] do you serve? You serve him.
Therefore: 'Whom do you serve' = CORRECT

Aren't there examples where this trick fails to work? Can you think of any?

I ran into this problem applying the trick to the question 'Who are you?'. I know, instinctively, that Who is correct, but if I were to use the trick, I would get it wrong:

[Who/whom] are you? You are him.
Therefore: 'Whom are you' = CORRECT

But this is obviously INCORRECT, so what gives? What am I doing wrong? What's the best way to remember how to correctly use Who/Whom

Answer 1

The trick does work substituting the objective pronoun or the subjective pronoun in place of "who/whom".

Did you speak to him?
Whom did you speak to?

Did you go to see her?
Whom did you see?

Who is it?

the problem you are running into is the "I/me" switch.
Properly, one should answer the phone

It is I

not

It is me

but the latter gets used very often and is understandable.

A way to remember "who" is subjective is that it's the name of the band

The Who - Who Are You?

imagine if they were named "The Whom"...?

Answer 2

You're right that this trick doesn't work all the time. Since whom is a formal pronoun whose use doesn't come naturally to speakers, it sounds wrong to use it with colloquial case rules. So this "trick" would fail to work in cases where prescriptive rules for case don't correspond to English speakers' intuitions. As you've said, one example of this is case after the copula. I can't think of any other common examples, but I did think of an uncommon one.

Consider the topic of the foliowing question: Tom is taller than I? Many people would say "Tom is taller than me." However, it would be wrong to use "whom" in a sentence like "Who is Tom taller than?" I don't think this is very useful information though because such sentences seem to be very rare. I only found one example so far, and it is from a translation of scripture, with a different word order and an archaic feel to it:

Stronger than who is he?

-The Two Targums of Esther, translated by Bernard Grossfeld

A more contemporary-sounding example I found uses "who" in a non-question context:

And when you have that many kids, there's always going to be a lot of competition - who's better than who, who's bigger than who, who's stronger than who and the way people usually figure that out is by proving it.

-Everyday Violence: How Women and Men Experience Sexual and Physical Danger, by Elizabeth Anne Stanko

Actually, this kind of construction—"who's [adj]er than who"—is probably the most common circumstance where "who" shows up as the complement/object of "than".

Another example I could think of is case in coordinate noun phrases (such as "Emily and I/me") but I don't think this is relevant to your question as "who/whom" seems unlikely to be used in coordinate phrases like this.

Answer 3

Put in mind that linking verbs or copular verbs do not require an object. Are/is/am, the verb to be is a linking verb. Unlike action verbs, linking verbs show a relationship between the subject of the sentence and a noun or adjective being linked to it. What comes after to be is a compliment and describes the subject. In other words to be does not make an action on the compliment, so a compliment is never an object. Who are you? I am a student. I am I. A student and I are compliments that describe the subject and not objects that receive the action. Therefore, you can' t say I am him. You are him is logically impossible. You are yourself not him. You are you. Who are you? Not whom are you? For more information, you can read about linking verbs and action verbs.