Comparison Gist and The W's Gist for GMAT | Verbal for GMAT PDF Download

COMPARISONS 

10% of GMAT SC Questions

All GMAT comparisons must contain items that are DIRECTLY comparable, and parallel.

Logical Match

These trigger words typically announce a comparison: like/unlike 

The GMAT test-writers will try to pull a fast one on you---the compared items won’t match. Find a pair of items that would make an exact logical match, and eliminate any answers that don’t involve a direct logical match.

Examples:  
Unlike the Beagle, the snout of the Bulldog...

Beagle and Bulldog match, but “Beagle” and “snout” don’t. It should be:

Unlike the snout of the Beagle, the snout of the Bulldog...

Or:

Unlike the Beagle, the Bulldog has a snout that...

You’ll also note that the items compared are also parallel. Comparisons and parallelism go hand-in-hand. 

THE W&s

10% of GMAT SC Questions 

Which vs. That

“Which” should be preceded by a comma, and modify the item just before the comma. A reverse modifier

Example:  
 Right: 
The world's largest steppe region, which is often referred to as "the Great Steppe", is found in southwestern Russia and neighboring countries in Central Asia,.

This is not to be confused with “in which”. “In which” describes detail within something. Example: “scenario in which”, “strategy in which”

When “which” appears all alone in the middle of the sentence, without a comma, almost always the solution is to use the word “that” instead.

Whether vs. If

“Whether” is used to introduce different outcomes/scenarios, and there is no need to include “or not”—it's implied. “If” is used to introduce a condition.

Example:  
Right (“whether” introduces two different outcomes: inventories large or even smaller): 
Factory orders are likely to decline in the months ahead, whether inventories remain fairly large, or even drop. “Whether” is correctly used here because it introduces different outcomes, not a condition.

Right (“if” introduces a condition: deal completion before Q3): 
If the deal were to be completed prior to the close of the third quarter, then we would likely earn a bonus.

When & Where

On the GMAT “when” can only be used in reference to a time, and “where” can only be used in reference to a place.

Example:  
 Wrong: 
The commander looked back to the phase in the initiative where the defensive line was weaker than it is today.

Right: 
The commander looked back to the phase in the initiative when the defensive line was weaker than it is today.

Who vs. That

“Who” refers to a person or a group of people. “That” is used to refer to anything else.

Examples:  
 Right: 
The leader who recently called for sales tax reform...
The boat that was used to break the speed record...