Put simply, who is used with the subjective (or nominative) case, while whom is used with the objective case. As you know, the subject causes an action, and the object receives the action:
Subjective: I, You, He, She, It, We, They, Who
Objective: me, you, him, her, it, us, them, whom
When we use who and whom correctly, it clarifies and emphasizes which case we are addressing:
subjective: Who gave you the contact? He gave me the contact.
objective: Whom did you give the contact to? I gave the contact to him.
subjective: Who called this morning? She called this morning.
objective: Whom did you call? I called her.
subjective: Who saw them at the beach? I saw them at the beach.
objective: Whom did you see at the beach? I saw them at the beach.
Tip: Notice how 'who' (one actor) is followed by a verb while 'whom' requires the auxiliaries do/did (because there are two actors).
Whom/Whom do you love?As you can see, who refers to the subjective case (I, you, he, she, it, we, they) while whom refers to the objective case (me, you, him, her, it, us, them). Now take a look at the examples from the beginning of this post:
This is the grammatically correct way to use who and whom, however nobody says "Whom do you love?" and everybody clicks on "Who to Follow". These days, whom is seen by some people as overly formal, uptight or stuffy, while others still use whom in spoken English, but generally speaking whom occurs more often in written English than in spoken. Whether one chooses to use whom or not, understanding English grammar is fundamental to speaking well.
In my opinion case markers, such as who and whom, enrich languages, adding specificity, clarity and at the same time, complexity to the expression of English. Old English was a much more complex language with far more cases, determiners and even pronouns. It's important to preserve these complexities in the English language, as it has become more watered down by the Internet, more mainstream as the world's international business language, all of which makes it one of the world's easiest languages to learn.