The wording of “main function” questions can be very misleading: after all, they inevitably ask about the main function of a paragraph in relation to the passage as a whole. The thing is, though, you don’t really have to really have to deal with the entire passage when trying to answer them. You don’t even have to deal with the entire paragraph that’s being asked about.
In general, you really only have to deal with a few key sentences: most often, the answer will be found in the first two sentences of the paragraph in question, although in some cases you may need to back up and read the last sentence of the previous paragraph.
As always, you should pay special attention to any major transition words (but, however, furthermore, etc.) or “interesting” forms of punctuation (semicolons, colons, quotation marks) that indicate the relationship between the preceding idea and the current idea.
To reiterate: The first sentence of the paragraph referred to in the question will often not give you the necessary information, so it’s important that you read the first two sentences. Normally the ACT asks about paragraphs that shift the focus from one idea to another, so be particularly on the lookout for anything that suggests contradiction.
Let’s look at an example:
2017/2018 practice test, section 3, question #16:
One of the main purposes of the last paragraph is to state that the:
F. gashes in the rift valley continue to increase in width.
G. seafloor of Atlantic has cooled.
H. entire Atlantic seafloor has issued from the gashes in the rift valley.
J. volcanoes on Earth’s dry land have created the newest, youngest pieces of Atlantic seafloor.
Strategy: The first thing we’re going to do is read the first two sentences of the last paragraph. We do not need to consider any other information.
Yet, what had seemed so foreign to scientists is an integral part of earth’s very being, for at the ridge our own planet gives birth. The floor of the rift valley is torn; from the gashes has sprung the seafloor underlying all of Atlantic.
It’s important to stress here that we don’t even need to know what’s going on in the passage to determine the function of the last paragraph. The paragraph itself provides all the information we need.
The first sentence doesn’t offer a lot of help, but the second sentence is key (note the semicolon). It tells us that the seafloor of the entire Atlantic has sprung from the floor of the rift valley, which is exactly what H says.