Who vs. Whom

The ACT tests over when to use who and whom.  For the most part, this isn't difficult...until your choice is in a subordinate clause.

Who is a nominative case pronoun used for subjects and predicate nominatives, and whom is an objective case pronoun used for objects.   Time  for a table:

Subjects and Predicate Nominatives (nouns)
Nominative  Case
All types of Objects
Direct, indirect, object of a prepostion
Objective Case

Before we complicate things with subordinate clauses, we can see that their use is similar to he and him, so try using he in place of who and him in place of whom in these examples.

(Who/Whom) is on the phone?  He would fit in the sentence, so we know who is the correct choice.
You don't like (who/whom?)   Him would work here, so the correct choice is whom.

Pretty simple.  However, the ACT doesn't typically test simple.  The complicated part is that who and  whom are both relative pronouns that can begin a subordinate clause. To brush up on subordinate clauses (AKA dependent clauses), please review the information at subordinate clauses.  Which one you pick is still determined by the role they play inside their clause.

To determine which one you pick inside a subordinate clause, you have to decide if the pronoun choice is going to be subject or predicate noun inside the subordinate clause or an object of some kind inside the subordinate clause. The key phrase there is "inside the clause."  The ACT tries to tempt you to look in the independent clause to determine  the pronoun choice because they're tricky that way.

I will place the subordinate clauses in bold in these examples so they stand out for you.

I know (who/whom) was in the play last semester
Okay, you're first instinct might be to look at I know and recognize the subject, I, with the action verb, know,  and think the pronoun choice will be a direct object, which would mean you must pick whom.   However, your second instinct is to remember my "look inside the subordinate clause" advice.  Inside the subordinate clause, the pronoun choice will be the subject for the linking verb was.  Consequently, you must choose who.

I know (who/whom) she chose.  Okay, using the inside the subordinate clause rule, you discover she is your subject and chose is your action verb, so your pronoun choice will be the direct object of chose.  Consequently, you have to choose the objective case pronoun  whom.  Yes, it's strange to see a direct object come before the subject and action verb, but that happens in subordinate clauses.

I know for (who/whom) the bell tolls.  I didn't bold the subordinate clause in this one yet.  Can you see where the subordinate clause begins?

Yes, it begins with the preposition for.  In all my examples, the subordinate clause is a noun clause acting as the direct object for know. Your pronoun choice will be the object of the preposition for, so you have to choose the objective form which is whom.   Of course, if you're a Hemingway fan, you recognized the title of one of his best books in the sentence and knew which pronoun to choose.  Being literate and loving books pays off sometimes.

So, what do you need to know to master who and whom?  A lot of the parts of speech including the subordinate clause. Loving grammar pays off big time on the ACT!