Reading comprehension is one of the two things GMAT Verbal now tests — sentence correction was removed in the Focus edition. The passages are shorter than the GRE's and the questions lean harder on structure.
Map the passage in four lines. As you read, hold: the main point, whose view is being reported, where the author disagrees, and what each paragraph adds. Write nothing long — a four-word note per paragraph. That map answers the main-idea and the function questions outright.
"Function" questions are the GMAT's signature. *Why does the author mention X?* You are not being asked what X is; you are being asked what job that sentence does in the argument — to illustrate a claim, to raise an objection, to concede a point, to introduce the exception. Ask "what would be missing if this sentence were deleted?" and the answer usually writes itself.
Inference means "must be true". A GMAT inference is not the most likely reading. It is what cannot be false given the passage. The option that goes one reasonable step further than the text is wrong every time, and it is designed to be the most attractive.
Tone is usually moderate. Academic authors qualify. An answer that makes the author enthusiastic or dismissive is usually too strong; the right one tends to be measured — "cautiously supportive", "skeptical of one aspect".
Watch the absolute words in the options. *All*, *never*, *only*, *impossible*. A passage that said "many" does not support an answer that says "all". Scanning the options for these words eliminates faster than re-reading the passage.