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 in question; that takes time, which you don’t have a lot of.
In general, you really only have to deal with three very important sentences: the last sentence of the previous paragraph and the first two sentences of the paragraph in question, focusing particularly on any major transition words (but, however, furthermore, etc.) that indicate the relationship between the preceding idea and the current idea.
The first sentence of the paragraph referred to in the question will often not give you the 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:
Official Guide, Test 3, Section 3, Question 17 (p. 469)
Question: The main function of the sixth paragraph (lines 58-64) in relation to the passage as whole is most likely to provide:
Strategy: The first thing we’re going to do is reread the last sentence of the previous paragraph and the first sentence of the current paragraph. We do not need to consider any other information.
Last sentence of paragraph 5:
“As author Daniel Chiras says, more companies are recognizing that ‘technologies that produce by-products society cannot absorb are essentially failed technologies.'”
First two sentences of paragraph 6:
“So far, we’ve talked about recycling within a circle of companies. But what happens when a product leaves the manufacturer and passes to the consumer and finally to the trash can?”
It’s important to stress here that we don’t even need to know what’s going on in the passage to figure out what the function of Paragraph 6 is. First, there’s no apparent relationship between the two paragraphs: 6 does not build on 5 in any way.
In addition, the phrasing “So far we’ve talked about…, but what happens…?” tell us that the author is summing up what he’s discussed so far and then moving the discussion in a new direction. So we need to look for an answer that says something along those lines.
F. evidence to support Daniel Chiras’s statement in lines 54-57.
No, it doesn’t even mention Chiras. As we said, there’s no relationship.
G. A transition between the two main points discussed in the passage.
Well, we don’t know how many points there are in the passage, but we do know that 6 is a transition that redirects the passage to discuss what is probably another very important point.
We’ll keep this as a possibility.
H. a conclusion to the author’s discussion about a no-waste economy.
It’s not a conclusion; it’s moving to a new idea. So C is out.
J. A summary of the author’s main idea
If you’d just read the first sentence of Paragraph 6, you might think this could work. But no, the author clearly indicates that (s)he is moving to a new idea. So while the author does summarize, that’s not the main point of the paragraph.
Which leaves us with (G).