Of all the three questions types on the GMAT Verbal section — Sentence Correction, Critical Reasoning and Reading Comprehension — SC is the question type that most people preparing for the GMAT tend to take a liking towards (it goes without saying that RC is the most hated). This fancy towards SC often leads to aspirants really digging deep into Grammar. So much so that they start spouting Grammar jargon! But there is more to GMAT Sentence Correction than the rules — a process or approach that will ensure that you do not become/remain a Grammar expert but become a GMAT SC expert.
This is a common and seemingly easily resolvable grammatical conundrum — when does one use which and when does one use that? Most test-takers who have prepared for GMAT® Sentence Correction will have this answer at the tip of their tongues — essential/restrictive and non-essential/non-restrictive clause. What they mean is that which is used to state information that is not essential while that is used to state essential information. A easier way to remember this is by looking at the pair of sentences below: 1. These are the keys to the fourth car in the parking row, which is black. 2. These are the keys to the fourth car in the parking row that is black. From the first sentence you would get the keys to the fourth car in the parking row; the sentence gives you some additional information, namely that it is black in colour; even without this information you could have known which car you have the keys to — the fourth car in the row.