Either as a relative pronoun or question word, whom is rarely used in conversation. Formal writing, yes, but ordinary conversation, no.
With whom did you go to the movies? is correct but sounds like a police interrogation.
Who did you go to the movies with? is technically incorrect but is the way we usually say it.
Is it better to be correct and absurdly formal, or incorrect and more accurate in mimicking native speakers, including the most educated? In ordinary conversation, generally speaking, the answer is you’re right to be wrong – use who.
Still, it’s a good idea to know which is correct in a given context. The quick test in choosing between who and whom is to substitute he or him. If he sounds better, who is correct; if him sounds right, whom is correct. That’s because as a pronoun whom is used to represent the object of either a verb or a preposition, while who represents the subject of a verb.
He is the consultant whom we contacted for advice. (We contacted him.)
To whom was the letter addressed? (The letter was addressed to him.)
He is the consultant who can answer your question. (He can answer your question.)
In the two examples above, the formality can be toned down by omitting the pronoun in the first, and using the more casual who in the second:
He is the consultant we contacted for advice.
Who was the letter addressed to?
The he/him test works well unless you’re confronted with a choice between whoever and whomever as in this sentence:
You can just talk with whoever/whomever answers the phone.
Even native English speakers get confused by this, because our instinct tells us that whom, not who, should follow the preposition with. However, there is another rule in English which dictates that every verb in a tense needs a subject. Here, whoever is the correct choice, since the verb answers needs a subject.
Might there be another situation when, besides addressing the Queen at a tea party, it makes sense to use whom as an object pronoun? If you see someone walking a dog, and the dog is so big and strong that it’s all its owner can do to keep up with it, you can ask, without risk of sounding ridiculously formal: Who is walking whom?