Even after using the word for many years, some of us may not have figured out yet that “either” actually functions in four ways:
When “either” functions as a pronoun or adjective, of course, the standard practice is to use it in the singular sense to refer to only one of two items, as in “Either of the two choices is unpalatable to me.” But when more than two items are involved, “either” becomes a semantic misfit, so we use the word “any” instead, as in “Any of these five runners is likely to win” (that’s “any” working as a pronoun) and “A red marking on any face of the cube will suffice” (that’s “any” working as an adjective). Some liberal teachers or editors might let us get away with using “either” in such sentences, but it is prudent to stick to “any” for our own semantic peace of mind.
The word “any,” though, simply won’t work as a conjunction in a form similar to “either…or.” This time, English gives us no choice but to break its rule of two and use “either…or” even if it refers to more than two items. Such is the case in the following “either…or” construction with three coordinate clauses: “Either you go ahead with your candidacy or we will field another candidate or we might as well forget about fielding one altogether.” With “either” right in front, the sentence can theoretically string more and more clauses with “or” and remain grammatically correct. Except for this minor semantic quirk, the behavior of “either” as a pronoun, adjective, and adverb is fairly straightforward and needs no further discussion.
In the case of “either…or” as a conjunction, however, we run smack against two sticky grammar conundrums: what form the verb and pronoun should take when the “either…or” construction mixes singular and plural elements, and how to execute the parallelism rule for grammar elements in “either... or” constructions.
Doing “either…or” constructions is simple when both of the elements referred to are singular or when both are plural. In the first case, the verb invariably takes the singular form, as in “Either my mother or my brother is coming tomorrow.” In the second case, the verb invariably takes the plural form, as in “Either my parents or my brothers are coming tomorrow.” But when we have a sentence that mixes singular and plural elements, should the verb be singular or plural?
In such mixed situations, an accepted practice is to make the verb agree with the number of the noun or noun phrase closest to it: “Either my mother or my brothers are coming tomorrow.” “Either my brothers or my mother is coming tomorrow.” But some grammarians feel that such mixed “either…or” constructions are inconsistent no matter what number the verb takes, so they suggest rewriting the sentence to get rid of the inconsistency. One way is to drop “either” and recast the sentence a little bit: “My mother or my brothers might come tomorrow.” “My brothers or my mother might come tomorrow.” The sense of both sentences is intact even with “either” gone, proof that the word is not functionally necessary in such mixed constructions.
All that remains is for us to make sense of the parallelism rule for “either…or” constructions. The basic principle in parallelism, of course, is matching in both form and structure all equally important ideas or grammatical elements in a sentence. One useful rule of thumb for achieving this in “either…or” constructions is to place the word “either” right before the first of the two elements being compared.