There are two ways of answering this question. First, easy answer: don't use whom. In typical informal English -- so 99% of spoken English -- "whom" almost doesn't get used. Second answer: "whom" is an indirect object pronoun. So it's used to designate indirect objects -- "To whom did you give the book?" (BUT -- as I said above -- everyone would really say "Who did you give the book to?') "Who" is a direct object pronoun -- "Who gave you the book?" Anyway, outside of grammar books and academic writing, "whom" is not used much.

Alejandro. Who significa "quién", whom significa "a quién". En el primer caso (who) es la persona que ejecuta la acción. ¿Quién vino? = who came? ¿Quién quiere más comida? = who wants more food? Mientras que en el segundo caso (whom) "quién" es el que recibe la acción: ¿A quién llamas? = whom are you calling? ¿A quién le tengo que decir? = whom do I have to tell? Ahora, también pueden usarse en frases positivas o negativas. Ella es María, quien está encargada de este departamento. This is María, who is in charge of this department. Él es Robert, a quien contratamos la semana pasada. He is Robert, whom we hired last week. Espero que esto te ayude

you dont have to, but in expressions like ' none of whom' the use of whom is indispensable.