Purpose
These questions will ask for why the author used a particular word or phrase -- key words in the question stem include "used to", "intended to", or "role". To do these, re-read the passage information, and before going to the choices, develop your own answer. This will allow you to see the subtle distinction that make choices correct or incorrect. See LSAT PrepTest 49, Section 3, Question 24, and LSAT PrepTest 50, Section 1, Question 17.
Definition
Sometimes the LSAT will simply ask you for the definition of a word or phrase, not the author's purpose -- key words/phrases in the question stem include "define" or "used to mean". These questions require that you be precise about the definition. So, before going to the choices, determine the definition first, looking for key points that must appear in a correct choice. As examples, see LSAT PrepTest 48, Section 3, Questions 8, 14, and 24; LSAT PrepTest 49, Section 3, Question 13; and LSAT PrepTest 50, Section 1, Question 27.