The term ellipsis, or elliptical construction, describes the omission of words in order to make a sentence more concise. Ellipses are used frequently with comparisons. An ellipsis may eliminate a subsequent occurrence of a word or words stated previously in a sentence only when the word or words are exactly the same each time they appear.

No ellipsis: During the 17th century, Britain experienced some types of political turmoil, and France, Spain, and Germany experienced others.
Correct ellipsis: During the 17th century, Britain experienced some types of political turmoil, and France, Spain, and Germany others.
The ellipsis omits experienced, which appears in this exact form earlier in the sentence. This ellipsis is correct.

Incorrect ellipsis: I walk as fast now as [I walked] when I was young.
This is not correct because “I walk” and “I walked” are not identical.
Correct ellipsis: I walk as fast now as I did [walk] when I was young.
In this case, “did” indicates the tense shift and stands in for “did walk.”

Incorrect ellipsis: I walk faster than Brian [walks].
Although we say this all the time in everyday English, this sentence is technically not correct. “Walk” and “walks” are not identical, so “walks” should not be omitted. Remember that a word or phrase can be omitted only if it exactly repeats a previous word or phrase.
Correct ellipsis: I walk faster than Brian does [walk].

In GMAT-land, the switch in verb tenses is not OK when you’re omitting verbs, but the switch in a comparison (I walk faster than Brian [walks] ) is sometimes acceptable if it is in the best option and it’s clear what the two things compared are.

On the GMAT, your job is to recognize when an ellipsis is not used correctly. An ellipsis can never be used when parallel construction is an issue, as it is in correlative conjunctions. For example, “I like to swim neither in pools nor lakes” is not correct. The item after “neither” starts with “in,” so the item after “nor” must also start with “in.” This is not a correct elliptical construction. If an ellipsis makes the meaning of the sentence unclear, then it is not correct. Take a look this sample question.

To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.

(A) have to know a great deal more than now about the social and economic
(B) have to know a great deal more than they do now about the social and economical
(C) would have to know a great deal more than they do now about the social and economical
(D) would have to know a great deal more than they do now about the social and economic
(E) would have to know a great deal more than now about the social and economical

First, it’s important to recognize that “have to know” and “would have to know” would both be correct here. They have different meanings of course, but each meaning could work. The “have to know” and “would have to know” split is pretty much just a diversion from the real errors in the sentence. We can immediately eliminate choices B and C; the word “economical” is not correct because its meaning is not logical in this context. We need the word “economic” to describe the “determinants of fertility.”

Next, we can eliminate choices A and E because they both contain ambiguous comparisons. Omitting “they do” is an incorrect ellipsis here because it changes the meaning of the comparison. The comparisons in A and E seem to compare how much demographers (would) have to know, an amount of knowledge, with the present time period (“now”).

D is the correct answer because it repeats “they do” to make the comparison clear.

Some of our best students have their grammar rules down pat. They can talk for hours about adjective clauses, dangling modifiers, gerunds, and the subjunctive, but they’re so busy checking to make sure that all the sentence parts fit into place that they forget to read the sentence for meaning. Consider this example:

Most studies approximate that 70 percent of individuals with an amputation experience phantom sensations in the amputated limb, often in the form of pain that is identical to the pain that they typically experienced when the limb was still attached to the body but contorted in an unnatural position.

(A) that is identical to the pain that they typically experienced when the limb was still attached to the body but
(B) that is identical to the pain that they typically experienced when that limb was still attached to the body but that was
(C) that was identical to the pain that they typically experienced when the limb was still attached to the body but was
(D) identical to the pain that they typically experienced when that limb had still been attached to the body but that had been
(E) identical to the pain that they would be experiencing when the missing limb was still attached to the body but when

Sometimes an answer choice will be wordy and awkward, and students will be tempted to dismiss it because they’ve been taught to look for clarity and conciseness. The trouble is that the other choices, though perhaps more elegant, distort the meaning of the sentence and make it illogical. The wordy and awkward choice is actually correct. This trick makes sentences like the one above especially difficult to navigate.

Got your answer? Let’s see how you did.

The original sentence, though slightly awkward, is grammatically correct. The sentence compares the pain caused by phantom sensations to a previous pain caused by a limb placed in a contorted position; the pain is identical to the pain experienced when the limb was attached but contorted… At first glance, the sentence might not appear to be in parallel form, but the different elements in the comparison accurately describe what they are supposed to describe. All the other choices introduce errors.

Choice B adds the word that and the verb was in an attempt to parallel the previous that with pain. However, this makes two parallel clauses about pain, and the pain…that was contorted makes no sense.

Choice C changes the first verb to was. Since the verb experience in the non-underlined portion of the sentence is in the present tense, the verb describing the pain must be in the present tense as well. The first part of the comparison is about the pain that is currently experienced; the second part of the comparison is about a past feeling of pain.

Choice D, like B, illogically describes the pain as pain…that had been contorted and incorrectly uses the past perfect had been, which must describe an event that took place before other past tense events in the sentence. However, the pain that they typically experienced was pain that took place at the same time that the limb was attached; the limb was not attached before they experienced the second type of pain compared.

Choice E uses would be experiencing to describe the pain that people actually experienced when a limb was attached to their bodies. Would be describes a hypothetical event, one that would be taking place if some other condition allowed for this event to occur.

Choice A is correct.

Takeaway: Instead of relying on conciseness alone, always keep in mind what the sentence is about. Intelligible sentences won’t always be grammatically correct, but be careful that you don’t veer too far in the opposite direction and discard wordy answer choices whose meaning is clear. Trap answers will often on the surface appear to be more “grammatical” than the correct answer. In many cases, though, they sneakily alter the correct meaning by masking it with awkwardness.