The title 'Who said what to whom?' really sums it up: who takes subject position and whom takes object position. But don’t get too carried away. Whom, although elegant sounding, is not always appropriate even when used correctly in the grammatical sense. What do I mean? Well, native speakers have a lot of extra information about a word that is not included in simplified rules. The information that I am talking about regarding whom is the level of formality in a specific social context, otherwise known as register.
There are contexts where informal language is most appropriate and contexts when formal is your best choice. You would use informal language, or a lower register, in chatting with friends or writing a note to your mum to remind her you’ll be home late, for example. You would use formal language, or a higher register, in essays, business emails, university lectures, and job interviews, to name a few. What does this mean for you? It means if you use whom, albeit correctly grammar-wise, while chatting with a co-worker at the coffee machine, you are likely to seem snobbish—it is out of place. And on the other side of the same continuum, if you fail to use it correctly in a dissertation, for instance, when the sentence calls for it, you may have lost the opportunity to prove that you do have a sound knowledge of English grammar.
Compare:
To whom shall I deliver this invitation, Sir?
Who do you want me to post this invite to, Ben?
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