It is definitely important to learn the rules of grammar when studying Sentence Correction. But as a GMAT instructor, I’ve noticed that some students try to apply grammar rules so ruthlessly that they sometimes do not understand how certain sentences can be correct. When we teach the past perfect tense, for example, we teach that the past perfect tense is used when a sentence contains two events that occur at different times in the past.  The past perfect should be used for the action that occurred first.

After I ate a lobster, I had gone swimming.

This sentence is incorrect, because the past perfect (indicated by the auxiliary verb – also known as “helping verb” – “had”) is used for the action that occurred second.  Many students would correct the sentence thus:

After I had eaten a lobster, I went swimming.

But this sentence is also correct:

After I ate a lobster, I went swimming.

I can anticipate my students’ protests: “But I thought you said that the past perfect is used when two events occur in the past, one before the other!” Well, yes, I said that — but I never said that the past perfect MUST be used. If the order of events is absolutely clear, as it is in the example immediately above, the simple past may be used.

In GMAT Sentence Correction questions, the grammar must be correct AND the logic must be clear. By focusing so much on grammar and neglecting to consider logic, many GMAT students miss correct answer choices because they incorrectly think there is a violation of a grammar rule, or they chose a grammatically correct answer choice that is not logically clear.

Here is another example:

The mountains of Peru, which are very high, are filled with diverse species of llama.

You maybe remember your teacher saying that “which” must modify the noun immediately before it. In that case, the sentence above would be wrong, because “which” clearly is intended to modify “mountains” not “Peru”. But the sentence is correct. The actual rule with “which” is that it must clearly modify a preceding, specific noun. The plural form of the verb after the “which” clearly indicates that it modifies “mountains.”

Some students, especially those who have a first language other than English, understandably want to learn a set of rules that can be applied consistently. The English language, however, tolerates more exceptions to its “rules” than many other languages, and the reasons for this are historical. English is a relatively recent language on the world scene, and it is a hodge-podge, a mixture of Anglo-Saxon, Norman French, Shakespeare (and, most recently, Sarah Palin). English was still evolving at a rapid pace when mass literacy emerged in the 16th century and forced a codification of its rules.  The “rules” that were established reflected English’s chaotic, fractured nature. When mass literacy came to other languages, these languages had already existed for much longer, and had a much more refined system of grammar in place.

In my next post, I’ll look at some actual GMAT Sentence Correction problems where a “rules-based” approach has gotten students into trouble. Till then, remember that logic can be just as important as grammar. If a sentence seems completely logically clear, but you feel there might be a modification, tense, or pronoun error, you might be trying to apply a grammar rule too rigidly.

In my last post, I covered the theory behind the Denial Test for Critical Reasoning Assumption questions and demonstrated how to do  negations properly. If you’re not sure what the Denial Test is or want a refresher on negations, be sure to check out that post before you read on.

All set?  Let’s now take a look at an actual GMAT Assumption question and see how the Denial Test can be very helpful. Try the question out for yourself first if you’d like, then read on for the explanation.

Diabetes is a disease in which the body does not produce or properly use insulin, a hormone that is needed to convert glucose into energy needed for daily life. Because ice cream commonly contains glucose in the form of sucrose, diabetics generally experience discomfort after consuming even a small serving of ice cream. However, since there are several ice cream companies who add sucrose to none of the ice cream they produce, diabetics are able to eat ice cream without experiencing discomfort due to glucose intolerance afterward.

Which of the following is an assumption on which the argument depends?

(A)These ice cream companies have been able to duplicate the taste of sucrose derived from glucose by means that do not involve adding any potential substances that may be of discomfort to diabetics.

(B) Not all forms of glucose are equally likely to result in this discomfort.

(C) Ice cream is not the only food to which glucose is commonly added.

(D)Apart from glucose, there are no substances commonly present in ice cream that would cause discomfort to diabetics.

(E) Glucose is not naturally present in the ice cream produced by these ice cream companies in amounts large enough to cause discomfort to diabetics who eat this ice cream.

Ready to find the answer?

First, let’s locate the conclusion in the question stem. The conclusion is the main clause of the last sentence (“diabetics are able…”). It is supported by the dependent clause that immediately precedes it (“since there are several ice cream companies who add sucrose to none…”), and the entire second sentence.

In general, negating each answer choice is not the most effective way to move through a question. It’s best to get down to two, maybe three choices, and then negate to find which one is a necessary assumption. But the purpose today is to learn more about negations, so we’re are going to look at the negation of each answer choice.

A) These ice cream companies have NOT been able to duplicate the taste of sucrose derived from glucose by means that do not involve adding any potential substances that may be of discomfort to diabetics.

This negated choice doesn’t hurt the argument, because the argument doesn’t focus on taste. Note that we negated the main verb, and touched nothing within the prepositional phrase (“by means that…).

B) ALL forms of glucose are equally likely to result in this discomfort.

If there is a quantity word modifying the subject of the sentence, we negate that instead of the main verb. The negation of “not all” is “all.” This negated choice does not destroy the argument, because the argument’s conclusion is about ice cream without glucose, not about different kinds of glucose.

C) Ice cream IS the only food to which glucose is commonly added.

This negation did nothing to the argument, because we are not concerned with glucose in other foods.

D) Apart from glucose, there are SOME substances commonly present in ice cream that would cause discomfort to diabetics.

“No” or “None” becomes “some” when negated. This negated choice seems to hurt the argument, but if we read the conclusion carefully, we realize it is only about discomfort from glucose, not discomfort in general. So D is incorrect.

E) Glucose IS naturally present in the ice cream produced by these ice cream companies in amounts large enough to cause discomfort to diabetics who eat this ice cream.

This negated choice destroys the argument, because it attacks the assumption that the added glucose is the only glucose in the ice cream. If you saw that assumption when we first read the argument, that’s great. But even if you missed it, the negation technique can help you to see clearly how an answer choice is a necessary assumption.

Keep practicing this technique and you’ll find it very useful on tricky Assumption questions.