It's a fine line though.... nearly all of us break grammatical rules of English to avoid sounding like a prat LOL

For example... most of us don't use the word "whom" we just use "who" instead even though "who" is meant to be reserved for a question. Most of us would say "who were you refering to?" instead "of whom were you refering?"
Actually who = subject and whom = object as in "who had sex with whom?"
Simple way to remember - if you can substitute s/he it is who and if you can substitute him/her it is whom - "she had sex with him?"