When I was in high school, my guidance counselor came to sub for one of our classes to talk to us about the ACT. Besides the little bit of prep that I did on my own with my sister's old ACT prep book, this was the only formal ACT preparation I ever had. The one thing that I remember about her overview of the ACT was her advice about commas:
"When in doubt, take it out."
I took that to heart and remembered it; I probably put it into practice whenever I took the ACT itself thinking that I had a cheat code for comma grammar questions (which are very frequent on the ACT English test).
However, what I have learned since then (which is my number 1 piece of advice about comma usage on ACT passages) is this:
"Commas, like other pieces of punctuation, are used deliberately."
Contrary to the beliefs of many high school students, commas aren't thrown willy-nilly into sentences to create artificial pauses. We pause often in our speech, but again, that doesn't mean that commas are needed when that same speech or thought is written down (even if it is a lengthy thought or sentence).
I'm going to invent a sentence right now about my coffee cup that needs no commas:
"The white coffee cup on the grey table at which I am sitting is adorned with a nearly solid black coffee collar that is designed to prevent my hand from being burned when I initially grab the delicious beverage from the barista behind the counter at my favorite coffee shop."
If I were to ask a student to locate where commas belong in this sentence, he or she would more than likely arbitrarily throw a comma in here or there wherever a pause would be nice. However, grammatically speaking, this sentence needs no commas.
On the other hand, I could write a sentence about my coffee cup that is much shorter but needs, grammatically speaking, a lot of commas, like this:
"Sitting on the table before me, the white, tall coffee cup is adorned with a coffee collar, which is designed to keep my hand, being sensitive to heat, from being burned, injured, or permanently damaged."
These two sentences are similar and communicate similar ideas, but the latter example needs deliberate commas from a grammatical point of view.
There are 4 ways that the ACT expects you to be able to use commas (and all 4 of them are used in the sentence above!):
1) To set off introductory clauses or adverbs ("Sitting on the table before me,")
2) To set off nonessential words or phrases (",being sensitive to heat,")
3) To set off adjectives in a series that describe the same noun ("white, tall coffee cup")
4) To set off items in a list ("burned, injured, or permanently damaged")
Though questions about numbers 1 and 2 are more frequent than questions about numbers 3 and 4, all 4 of these rules regarding commas need to be mastered to elevate an ACT English score.
In my online course The ACT System or my book The ACT English System I offer this kind of thorough English preparation in a systematic, research based way. In fact, mastering commas is, according to my own research into previously administered ACT English tests, "Step 3" out of 7 in mastering the ACT English test.