The word "who" is followed by its own verb:
I don't know who is responsible for this mess!
In this example, "who" has the verb "is." The word "whom" is preceded by a verb or by a preposition that has captured it (that is, it is the "object" of a verb or a preposition).
To whom will you direct this question?
Whom do you trust? (Actual sentence structure: "You do trust whom?") That's all. Simple.
Failure to use these two pronouns correctly will result in a much lower grade. Also, don't use "that" as a substitute for "who" or "whom." This subterfuge won't work.
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