First of all, try this GMAT Sentence Correction practice problem.
1) Knocked from the asteroid belt, an asteroid that comes close to Earth may be captured by Earth's gravitational field, ultimately spiraling inward and, fully consumed during its fiery descent through the atmosphere while being a "falling star", or is redirected at high speeds along a new trajectory.
- spiraling inward and, fully consumed during its fiery descent through the atmosphere while being a "falling star", or is redirected
- having spiraled inward and, fully consumed when its fiery descent through the atmosphere to be a "falling star", or is redirected
- having spiraled inward and being fully consumed when its fiery descent through the atmosphere as a "falling star", or was redirected
- spiraling inward and, fully consumed during its fiery descent through the atmosphere to act like a "falling star", or be redirected
- spiraling inward and being fully consumed during its fiery descent through the atmosphere as a "falling star", or be redirected
A full explanation of this question will come at the end of this post.
An auxiliary verb, sometimes called a helping verb, is a verb that comes before a main "action" verb to indicate tense or some other verb quality. Here is an assortment of different auxiliary verbs with the verb "to sing" (I deliberately chose a verb that would have a different spelling in the past tense vs. the past participle). All auxiliary verbs are underlined.
1) I sing X. (simple present tense)
2) I sang X. (simple past tense)
3) I will sing X. (simple future tense)
4) I am singing X. (present progressive tense)
5) I was singing X. (past progressive tense)
6) I will be singing X. (future progressive tense)
7) I have sung X. (present perfect tense)
8.) I had sung X. (past perfect tense)
9) I will have sung X. (future perfect tense)
10) I have been singing X. (present perfect progressive tense)
11) I had been singing X. (past perfect progressive tense)
12) I may sing X. (expresses permission or hypothetical possibility)
13) I may be singing X. (subjunctive mood, present progressive tense)
14) I might have sung. (subjunctive mood, present perfect tense)
15) I can sing X. (expresses ability)
16) I could sing X. (expresses hypothetical ability, ability under certain circumstances)
17) I could have sung X. (hypothetical ability in the past)
18) I would sing X. (subjunctive mood, future tense)
19) I would have sung X. (subjunctive mood, past tense)
20) I should sing X. (expresses strong expectations)
21) I should have sung X. (strong expectations in the past)
22) I must sing. (expresses necessity)
23) I must have sung. (necessity in the past)
24) I do sing X. (expressive affirmative emphasis)
25) I did sing X. (affirmative emphasis in the past)
I will not guarantee that this is an exhaustively complete list, but this certainly accounts for the vast majority of auxiliary verbs you will see on the GMAT.
The number of a verb concerns whether the verb is in the singular or plural. In practice, the number of a verb only matters in the third-person. Consider the present tense of the verb "go":
This is emblematic for almost all verbs in the present tense: the five cases (first & second persons singular & all three plural cases) are identical, and only the third person singular case differs. The only verb in the entire English language that is not identical throughout those former five cases is the highly irregular verb "to be" (I am, you are, he is, we are, …). Notice that most of the topics on the GMAT Sentence Correction are academic topics, say science or business, and essentially every sentence on the Sentence Correction section will be in the third person, in which verb number matters.
Notice, also, that unlike virtually all ordinary verb, many auxiliary verbs (will, can, may, should, could, would, etc.) are identical in all six cases and show no difference in form from third person singular to third person plural. Only a few auxiliary verbs (is/are, has/have, does/do) change form when number changes.
Number and Auxiliary Verbs
When an ordinary verb is preceded by one or more auxiliary verbs, only the first verb in the sequence would change form for a change in number. Only the first verb would show singular or plural, and if the first verb is an auxiliary verb that, like most auxiliary verbs, doesn't change form with number, then nothing about the verb will reflect a change in number. Here are some examples of singular/plural pairs in a variety of different forms.
1a) He will go.
1b) They will go.
2a) He can go.
2b) They can go.
3a) He had gone.
3b) They had gone.
4a) He would have gone.
4b) They would have gone.
5a) He will be going.
5b) They will be going.
6a) He does not go.
6b) They do not go.
7a) He has gone.
7b) They have gone.
8a) He is going.
8b) They are going.
9a) He was going.
9b) They were going.
10a) He has been going.
10b) They have been going.
In examples #1-5, the leading verb was identical in both the singular and plural, so the entire verb has the same form in the singular and the plural. In examples #6-10, the leading verb changes with changes in number, but everything after the leading verb remains the same.
A technical note: in #1, #2, and #6, the verb "go" is not in the present tense plural form, but in something called the infinitive form, the form the verb would take after the word "to" in an infinitive. The infinitive form is the main form of any verb: all verbs are listed in the dictionary by their infinitive form. For most ordinary verbs on the planet, the infinitive form is identical to the third person present plural (to do, they do; to have, they have; etc.); in fact there is only one verb in the entire language which is an exception to this rule: to be vs. they are.
A curveball: parallelism
As well all know, parallel structure is one of the GMAT's favorite syntactical forms. On any GMAT Verbal section, at least half of the Sentence Correction questions will involve some kind of parallel structure.
Parallel structure can make the above rules about auxiliary verbs & number harder to follow. For example, consider this form of a sentence
[singular subject] may not only have X (modifier)(subordinate clause) but also ("do"/"does") Y.
Suppose part of the split in a GMAT Sentence Correction question is determining the number ("do" vs. "does") for that verb after the "but also". The mistake folks will make is to look back at the singular subject and think --- well, if it's a singular subject, the verb has to be singular as well: "does". That's a mistake, because the auxiliary verb "may", before the "not only", applies to both terms --- "may not only P but also Q". Common words outside the parallel structure apply to both terms. The first verb, "have", technically is not in the plural form but in the infinitive form, because it follows the auxiliary verb "may". Therefore, the second verb also must be in the infinitive form, "do." If you notice, the practice Sentence Correction question at the head of this article has precisely this kind of split!
Having read this article and noticed this hint, go back to that sentence and give it another try before reading the solutions below. Here's another practice Sentence Correction question that, in testing different tenses, involves a variety of auxiliary verbs.
Practice question explanation
1) Split #1: the parallelism involving auxiliary verbs & number. Eliminating the fluff, we have "an asteroid … may be … or (?)" Singular subject, so folks will be tempted to put a singular verb, "is" or "was", after the word "or." Both parts of the "or" structure are under the same hypothetical structure: the asteroid may do one thing or may do the other thing. The word "may" is not repeated, because there is no need to repeat common words in parallel structure, but following that auxiliary verb, we need the infinitive form --- not "is" or "was", but "be." Only choices (D) & (E) are correct on this split.
Split #2: "during" vs. "while". The word "during" is a preposition, and need be followed only by a noun as its object: "during its fiery descent" is a complete prepositional phrase. The word "while" is a subordinate conjunction, which must be followed by a full [noun]+[verb] clause: choices (B) & (C) use the word "when", but neither of these have a bonafide [noun]+[verb] structure following the word "when", so both of those are incorrect.
Split #3: look at what follows the word "atmosphere", the part that discusses the idea of a "falling star"
(A) … while being a "falling star"
(B) … to be a "falling star"
(C) … as a "falling star"
(D) …to act like a "falling star"
(E) … as a "falling star"
The first phrase, "while being", is an atrocious abomination --- that would never be correct on the GMAT. (A) is wrong. An infinitive "to be" could be correct, but we would need something before this either to require the infinitive or at least to set up the infinitive, and nothing does this, so the (B) is wrong. The concise structure "as a falling star" is perfectly acceptable, so (C) & (E) handle this correctly. Choice (D) involves a very interesting idiom: "act like" ---- idiomatically, the verb "to act like" means the agent consciously chooses to imitate something. People can "act like" something, and arguable even some intelligent animals can "act like" something, but inanimate objects can't. The correct idiom for inanimate objects is "act as", which is not option in this sentence, and would change the meaning anyway. The idiom "act like" is incorrect in this context, so (D) is wrong.
Split #4: participle parallelism. Choices (C) & (E) correctly have two participles in parallel: "spiraling … and being fully consumed …" That's correct. The other three choices put a comma after "and", turning "fully consumed" into a modifier, which creates the bizarre structure "spiraling … and, [modifying phrase], or" which is awkward and incorrect.
The only possible answer is (E).
This post was written by Mike McGarry, GMAT expert at Magoosh, and originally posted here.