Both the ACT and the SAT test this grammar rule.
The official explanation
There are official explanations of the difference between the two. (Erica Meltzer has a explanation of an exception over on her blog, the Critical Reader.)
The pronoun “who” is used as the subject of a sentence. (Who is doing the action?)
Who threw the ball?
The pronoun “whom” is used as the object of the verb (Whom is receiving the action?)
I threw the ball to whom?
But most of my students can’t reliably distinguish between a subject or an object in a sentence.
The easy way
There’s a much easier way that works 99% of the time.
Substitute HE or HIM into the sentence. If HE works then use WHO. If HIM works then use WHOM. tell a friend
If “he” works, then use “who”
Who threw the ball? He threw the ball. (“Him threw the ball” doesn’t make sense.) So “who” is the correct pronoun to use.
If “him” works, then use “whom”
Which sounds better? “I threw the ball to he” or “I threw the ball to him”? “He” doesn’t make sense but “him” does. So the correct pronoun to use is “whom.”
(Notice them “m” in him – add that to the who to create whom)
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