"Mr. Martin, Mr. Martin!" the panicked teenager helplessly cried out, "I still can't get it...how do I know when to use 'who' and when to use 'whom'! Can you help?"
This young person was putting too much pressure on himself, for one thing, but I did my best to explain the difference and calm his nerves. However, that wasn't the end of the lesson. I then wen't on to explain that it isn't even guaranteed that there will be a question requiring knowledge of who vs. whom on his ACT English test that he was last-minute-panicking about.
It is true that there are a handful of small English rules (I identify roughly 15 in my book The ACT English System) that combine for roughly 3-6 questions on each ACT English test. Panicking about these small rules is unlikely to be too productive. Don't get me wrong, netting an additional 3-6 questions on your next ACT English test would be great, but to do so, you will have to master these 15 or so small English rules. There is a time for that (for example, you're making in the 30's on your ACT English test and want to go higher), but it isn't the way to start.
This is why my strategy in ACT prep has been to put first things first, and by that I mean preparing in order of importance or in order of most-tested skills. Would you rather spend your time trying to memorize and master 15 rules to get an additional 5 questions correct, or would you rather spend your time trying to memorize and master 5 rules to get an additional 30 questions correct?
Of course it's not that easy; if it were, everyone would ace the ACT. But, it is a great place to start.