These two types of questions are both quite common, and both quite similar. As such, try to do them one after the other (if both appear for a passage). The first question of most passages asks for the main point. Primary purpose questions tend to be the final question -- see the third passage of LSAT PrepTest 50, Section 1.
Main point questions ask you for the content of the passage, and will usually provide details from the passage in the choices. Primary purpose questions ask you for the author's intent. Quite simply, main point questions ask you what is in the passage, whereas primary purpose questions ask you why the author wrote it. These questions require that you read the entire passage -- do not read only the first paragraph assuming that a clear thesis statement will appear, or read only the final paragraph assuming that a clear conclusion will appear. Also, don't try to intuit the main point or purpose -- go to the passage and find explicit information.
If you're having difficulty determining main point or primary purpose, try creating paragraph labels of a maximum of four words, which forces you to get at the heart of the paragraph, and, forces you to use general language, which will benefit you with the answer choices later. Paragraph labels for LSAT PrepTest 50, Section 1, Passage 2 might look like this:
You may wish to do the labels after reading each paragraph, or do them after reading the entire passage. Although creating these labels might seem to take up too much time, recognize that once done, you will have a significantly easier time recognizing correct and incorrect choices, and, thus their use will likely lead to your spending less overall time doing main point and primary purpose questions. Additionally, the labels will help you locate information for other questions as well.
Main Point
The first question of an LSAT Reading Comprehension section will typically ask for the main point of the passage -- what is contained within. As noted above, use the paragraph labels to determine the main point beforehand, then go to the choices.
Once to the answer choices, be wary of choices that only discuss part of the passage, but not the entire passage. Incorrect choices like this will commonly describe one of the facts that an author has presented in support of his or her argument, but will not contain the author's opinion. See (B) on LSAT PrepTest 50, Section 1, Question 6.
Also be wary of choices that are much too broad in scope.
Be wary of answer choices that sound really good but don't accurately describe the main point. As an example, see (E) for LSAT PrepTest 50, Section 1, Question 1. The choice contains some great adjectives -- "powerful" and "organic" -- but the author never uses these terms or similar terms, and the choice does not describe the main point.
"Title" Questions as Main Point Questions
On occasion, the LSAT will contain a question that asks you for a title that describes the passage's content. When people create titles, they often create "catchy" titles that will lure potential readers. However, note that the LSAT only asks for a title the describes the content's passage. As such, you want to focus purely on content -- how a title "sounds" means nothing. Thus, you may encounter an answer choice that sounds "catchy" but is incorrect; or, you may encounter an answer choice that sounds boring, yet accurately describes the passages content and is the correct choice. Also, never choose an answer choice because it "sounds impressive" because it is long and contains dense vocabulary. Consider PrepTest 49, Section 3, Question #7. Choices (B) and (D) both sound catchy and impressive, but are nonetheless incorrect. The correct choice, (A), sounds a bit dull but accurately captures the passage's content.
Primary Purpose
These questions ask why the author wrote the passage at all. The choices will usually contain general language, so, do not seek details in choices. See LSAT PrepTest 49, Section 3, Question 10.
Also, the choices often begin with verbs, and, focusing on these verbs can greatly assist you to eliminate incorrect choices. See "classify" in (A) of LSAT PrepTest 49, Section 3, Question 10.