Day 259: Either is single.
Should you write “Either of these books is fine” or “Either of these books are fine”? In these two sentences, “either” is the subject. The main verb is either “is” or “are.”
Either is singular, being short for either one. Because either is singular, you need the singular verb is.
In the sentences above, you are stating “This book is fine, and that book is also fine.” You are referring to only one book at a time, which also helps explain why you need the singular verb. Consider this sentence.
“The client informed us that either of the two revisions [meet/meets] his needs.”
Here, “either” is the subject of the verb “meet” or “meets.” Again, “either” is singular, referring to only one, or one at a time, so we need the singular verb. The solution, therefore, is as follows.
“The client informed us that either of the two revisions meets his needs.”
