Correlative pairs such as either…or, neither…nor, not only…but also, and whether…or also require parallelism. When you see one of these pairs in a sentence, check to make sure that the words or groups of words immediately following each conjunction are in the same form.

Consider the following sentence:

Either I will attend the show, or they will be attending.

This sentence uses the correlative pair “either…or” to present a set of two options. Are both in the same form? Compare the structures immediately following each conjunction:

(Either) I will attend: pronoun + future-tense verb

(or) they will be attending: pronoun + future-progressive-tense verb

Both constructions use a pronoun followed by a verb, but the verbs do not match. Parallelism dictates that both verbs must be in the same form:

Either I will attend the show, or they will.

OR

Either I will be attending the show, or they will (be attending).

The first version has two future tense verbs, while the second version has two future progressive tense verbs. Both tenses are appropriate for describing an event of some duration that will take place sometime in the future.

Incorrect: Either I will attend the show, or they will be attending.

Correct: Either I will attend the show, or they will.

Correct:
Either I will be attending the show, or they will (be attending).

Both latter versions are correct.

Watch out for matching clauses or phrases with single words.

Consider the following sentence:

Not only has the captain assigned all his men to the case, but also a private detective.

This sentence reads well at first glance, but it contains a hidden grammar error.

Compare the structure of the groups of words following each conjunction in the “Not only…but also” pair:

(Not only) has the captain put all his men on the case: clause

(but also) a private detective: noun

These two structures definitely do not match. A better way to write this sentence is:

The captain has assigned to the case not only all his men, but also a private detective.

Here, “not only” and “but also” are both followed by nouns: “men” and “private detective.”

Alternatively, both can be followed by phrases:

The captain has not only assigned all his men to the case, but also hired a private detective.

Here, “not only” and “but also” are both followed by verb phrases: “assigned all his men” and “hired a private detective.” Note that both verbs must be in the same form.

Final tips on recognizing parallelism
Look for:

Lists
Correlative pairs
Comparisons using multiple pronouns

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