English language is tricky at times. Understanding what word to use in a particular context can be challenging. You need to read a lot of English literature, preferably classics, to make sense of all the subtleties of the English language. Let's look at the very close pronouns who and whom. Essentially, who and whom are very similar. In modern-day English, who is often used instead of whom. That being said, there are differences in when to use whom and who.

When to Use Whom And When to Use Who

Use who when you want to say "who" did the action (some subject; e.g., Nick, Paul, or Alexandra). Use whom when you want to talk about a person or a thing when it is not a subject but an object. Look at the following examples:

Who asked you that? (who is the subject) as opposed to "Whom did you ask?" (you is the subject, whom is the object)

Whom did you see tonight?

Whom did he appoint as his assistant?

Whom did he give the papers?

Whom do you like most?

Whom did John punch yesterday?

In short, whom is used when we want to talk about an object, and not as a subject, whereas who is used when the talk about a subject, the doer of the action.

Sources:

Wikipedia: Who (Pronoun)

Who is Correct

Wiktionary; Whom