I think it’s fair to say that the ACT really, really likes to test commas. I’ve never done a statistical analysis, but I’d wager that it’s around 15-20%.
But while the ACT does test commas in many different ways, the reality is that you don’t have to know every last rule governing comma usage. As long as you know the major ways in which commas are used correctly, you can probably identify when a comma is not being used correctly.
So that said, here are the contexts tested on the ACT that require you to absolutely, conclusively use a comma:
1) Before a FANBOYS (coordinating) conjunction when joining two independent clauses
Example: London is a very old city, and it contains buildings from many different eras.
2) To set off a non-essential clause that can be removed from a sentence
Example: London, which is a very old city, contains buildings from many different eras.
3) Between items in a list
Example: London contains buildings from time periods including the Middle Ages, the Renaissance, and the Victorian era. The third comma is optional, by the way. Since the sentence is correct both with and without it, you’ll never be tested on that particular usage.
4) To separate multiple adjectives whose order could be reversed
Example: London contains many interesting, eclectic neighborhoods, OR London contains many eclectic, interesting neighborhoods.
When you see a comma, ask yourself whether it’s being used in one of the above ways. If it isn’t, you can be relatively certain that you should choose an answer that doesn’t include it.