Comparisons are a special case of parallelism. A number of comparison-specific constructions call for you to always express ideas in parallel form. These constructions include:

Either X or Y...
Neither X nor Y...
Not only X but also Y...

X or Y can stand for as little as one word, or as much as an entire clause, but in every case, the grammatical structure of X or Y must be identical. For example, the sentence Either drinking or to eat will do violates the rule by mismatching verb forms:

This is a comparison, and requires parallelism. Both verbs must be in the same form: because they aren't currently in the same form, one must be adjusted.

Both verbs are now in the –ing form. Though in many cases of parallelism either verb form is fine, for Either/Or comparisons such as this one, both verbs must be in the –ing form.

Here's another example, using Neither/Nor:

Neither an interest in history nor to be adept in a foreign language is going to help you learn to sing.

This sentence lists two talents one could possess, in a neither/or format. They are not, however, in the same form.

In this sentence, a noun is compared to a verb. Though it's a different kind of mistake than the missing-information and verb-form errors we've looked at, it should be dealt with in the same way: by changing one of the forms to match the other.

Both phrases are now in the same form: "an interest in" and "an adeptness in". In this instance, the verb had to be changed to match the noun, instead of the other way around, because "to be" verbs don't belong in comparison (either/or, neither/nor) sentences.

Incorrect: Neither an interest in history nor to be adept in a foreign language is going to help you learn to sing.

Correct: Neither an interest in history nor an adeptness in a foreign language is going to help you learn to sing. .

A. Introduction

C. Comparative and Superlative Forms