There is only one exception: The verb to be requires the use of who
regardless of where it appears in the sentence. For example, in analyzing
whether to replace him (or her) with who or whom in the following
sentence, use the three-step model:
I know him. (or “her”)
1. The verb is “know.”
2. The subject is “I.”
3. The word him (or her) is not the subject of the sentence; therefore,
it is correct to use whom.
I know whom?
But now consider replacing the word he (or she) with who or whom
in this sentence:
I am he. (or “she”)
1. The verb is “am.”
2. The subject is “I.”
3. The word he (or she) is not the subject of the sentence; therefore,
it would seem to be correct to use whom. However, the word am
is a form of the verb to be and as such is the exception to the
rule. Thus it is correct to write:
I am who?
Use this three-step process to analyze sentences until you have trained
your ear sufficiently to make the determination automatically. If you’re
alert for the one exception (any form of the verb to be), you should find
it easy to make the who/whom determination correctly every time.
Collective Nouns Are Singular When Acting as One Unit
Remember to watch for collective nouns (discussed earlier), words referring
to groups of people or things. Notice that in every example that
follows, the pronoun is singular.