I can't say I care for the advice in the link above; it erroneously suggests that the grammar of "who" vs "whom" is parallel to that of "he" vs "him". But that is wrong; although "whom" is an objective form of "who", "who" is often used as an object, whereas "he" is not. You will find more reliable advice at Merriam-Webster, which says (http://www.merriam-webster.com/dictionary/whom): Full Definition of whom objective case of who —used as an interrogative or relative ; used as object of a verb or a preceding preposition <to know for whom the bell tolls — John Donne> or less frequently as the object of a following preposition <the man whom you wrote to> though now often considered stilted especially as an interrogative and especially in oral use ; occasionally used as predicate nominative with a copulative verb or as subject of a verb especially in the vicinity of a preposition or a verb of which it might mistakenly be considered the object <whom say ye that I am — Matthew 16:15 (Authorized Version)> <people…whom you never thought would sympathize — Shea Murphy> (In other words, unless "whom" follows a preposition (e.g. "the man to whom I gave the book"), it is not much used in colloquial English, and it is reserved for formal expression.)

This may help ! http://www.grammarbook.com/grammar/whoever.asp