One of the most-asked questions we get is whether to use a singular or plural verb with the word "or." Here is some helpful information.
Use the singular form of the verb when two or more subjects are connected by or--with the following exceptions: (1) when all the subjects are plural; (2) when the subject nearest the verb is plural.
- Either Sally or Weeze is going to be with us for the water ski trip to Lake Powell. (The verb is singular because both subjects joined by or--Sally and Weeze--are singular.)
- I predict either the Phillies or the Marlins are going to win the NL East title. (The verb is plural because both subjects--Phillies and Marlins joined by or--are plural.)
- Either Mervin or the twins are going to be here for the festivities. (The verb is plural because twins, the subject closest to the verb, is plural.)
- Either the twins or Mervin is going to be here for the tournament. (The verb is singular because Mervin, the subject closest to the verb, is singular.)
When the nouns or pronouns that make up a compound subject represent different persons (first person [I], second person [you], etc.), the form of the verb is governed by the person of the word nearest the verb.
- Either Ted or I am going to judge the ski jumping contest. (The choice of am over is is governed by the fact that I [first person: am] is closer to the verb than Ted [third person: is].)
Source: Grammar for Smart People, by Barry Tarshis.