Recently, you told me I was wrong for using whom, when in fact I was correct for using it.You will never be wrong if you always use who, and never whom. Whom is dying out. You can show off your expertise by learning to use it correctly, but most native speakers of english can't do so unless they're over 50 or have studied english formally, so only the educated will even know you got it right.
To figure out when to use which, try to rewrite the sentence with he and him instead of who or whom. Anywhere you would use him is safe to use whom.
Traditional grammar says that the nominative form should be used where a personal pronoun is complement of "be". But there is no such rule: the verb "be" can take an accusative (objective) case pronoun. For example,Why we do not write "Whom I am?" but we write "Who I am?" Beacause after "am" is an object, so we should use "whom"?