On the road these summer days, you might encounter a very confusing car magnet. It is confusing not in its message but in its grammar.
“Who” is a subject pronoun, meaning it should only replace the subject of a sentence – in this case, the person or animal doing the rescuing. It cannot replace the object of a sentence – in this case, the person or animal being rescued. What is needed is an object pronoun, namely, “whom.” And so the magnet should read, “Who rescued whom?”
“Who” and “whom” are a source of endless confusion in a way that other pronouns, such as “he” and “him,” are not. Most of us would not say, “Did him rescue he?” No, “he,” the subject pronoun, is the initiator of the action and, in this construction, must comes first. “Him,” the object pronoun, is on the receiving end of the action and, in this construction, must come second.
One way to remember the distinct roles that “who” and “whom” play in English grammar is to establish a mental association between “whom” and its fellow object pronoun “him,” both of which end in the letter “m.” Another indicator that “whom,” not “who,” is called for is the presence of a preposition, as in, “By whom was he rescued?” Prepositions are always followed by object pronouns, but the answer to this question can also be our guide: “He was rescued by him,” and “him” and “whom,” as you now know, stick together.
I do not expect to see a mass revision of ungrammatical car magnates, but the next time you are printing something for mass consumption, think about the crowd to whom you are speaking.
For some additional – and colorful – advice on “who” versus “whom,” check out this site.