When the pronoun either
is the subject and comes immediately before the verb, the verb is singular: Either is good enough. Either grows well in this soil.
is followed by a prepositional phrase with a plural object, there is a tendency to use a plural verb, but a singular verb is more common: Either of them is (or are) good enough. Either of the shrubs grows (or grow) well in this soil.
As an adjective either
refers only to two of anything: either side of the river; using either hand.
As a pronoun either
sometimes occurs in reference to more than two ( either of the three children
), but any
is more common in this construction ( any of the three children
). As a conjunction, either
often introduces a series of more than two: The houses were finished with either cedar siding or stucco or brick. The pizza is topped with either anchovies, green peppers, or mushrooms.
Usage guides say that the verb used with subjects joined by the correlative conjunctions either
) is singular or plural depending on the number of the noun or pronoun nearer the verb: Either the parents or the school determines the program. Either the school or the parents determine the program.
Practice in this matter varies, however, and often the presence of one plural, no matter what its position, results in a plural verb: Either the parents or the school determine the program.
In carefully edited writing, these correlative conjunctions are usually placed so that what follows the first correlative is parallel to what follows the second: The damage was done by either the wind or vandals or either by the wind or by vandals (not done either by the wind or vandals).
See also neither