There’s a special subject-verb agreement rule that you should use in either or sentences. (Click on the link and read Rule 3, or just keep reading below.)
Here’s the rule: Pretend the “either” part of the sentence isn’t there. Look only at the or part.
Here’s an example:
Either several cars or a mini-bus (is/are) needed to transport the officers to the parade.
Skip “Either several cars” and go straight to “a mini-bus.” The correct word will be is.
Either a several cars or a mini-bus is needed to transport the officers to the parade. CORRECT
(Incidentally, this is one of very few times you don’t look at the beginning of a sentence to get the verb right.)
Here’s another one for you to try. The answer appears below.
Either Mrs. Jones or her children (was/were) probably home when the burglary occurred.
Skip “Either Mrs. Jones” and go straight to “her children.”
Either Mrs. Jones or her children were probably home when the burglary occurred. CORRECT
One more thing: Neither/nor sentences work the same way.