What's the easiest way to explain who and whom to a Spanish speaker?
Determine la función de la palabra (who o whom) que falta en la oración o cláusula. Si se trata del sujeto de un verbo, úsese "who"; si se trata de un objeto del verbo o de una preposición, úsese "whom".
Sujeto: John is the man who is speaking now. "who" es el sujeto del verb "speaking".
Objeto del verbo: I saw him at the store. "him" es el objeto directo del verbo "saw".
I gave him the book. "him" es el objeto indirecto del verbo "gave".
Objeto de una preposición: He is the man whom everyone is talking about. (Más formal: He is the man about whom everyone is talking.) "whom" es el objeto de la preposición "about".
Cuando hay duda, úsese "who". Nadie, salvo un pedante, se quejará.
"Who cares" isn't a good example because it doesn't translate literally. In English it requires a subject pronoun and verb, while in Spanish it requires an object pronoun. "A quien le importa" is more like "to whom does it matter", but that would sound rather odd in English.
This page may help:
Candice, that was exactly my point. "Who cares" doesn't work as a comparison because it isn't a literal translation of "a quien le importa". The Spanish equivalent doesn't use a subject pronoun. "Who" is a subject pronoun and "whom" is an object pronoun. The difference is the same as the difference between "he" and "him".
My advice would be to forget about 'whom'. You really don't need it.
I thought the websites would be helpful in explaining who and whom. Neil's explanation is also very good and very similar to what the websites I provided say. My first response was addressing your question about "who cares" and why that wasn't a good example. "A quien" usually is translated as "whom" or "to whom."