Who is the subject. Whom is the object.
This means that if you are asking about the subject of a sentence you use "who". Such as "Who is shopping?" would be answered with "He, she or they are shopping."
If you are asking about the object of a sentence you use "whom". Such as "I am shopping with whom?" would be answered with "I am shopping with her, him or them".
Another way to remember is the difference between who and whom is the same as the difference between he and him
You'd say "he is shopping" not "him is shopping".
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