First, let’s review the basics of making compound subjects agree with verbs. When two or more subjects are joined by “and,” then they are almost always a plural subject and so take a plural verb.
Example: My cat and dog greet me at the door when I come home.
The exception is when the compound subject is a single unit, such as “research and development,” which would then take a singular verb.
When two or more singular subjects are joined by “or” or “nor,” then they take a singular verb. If the subjects are plural, then they take a plural verb.
Examples: Either Joe or Leah goes shopping for their mom.
Neither potato chips nor hot dogs are healthy foods.
The Proximity Rule comes into play when one of the subjects is singular and one is plural. Look at the subject nearest the verb to determine if the verb should be singular or plural.
Example: Either her brother or her sisters were responsible for leaving the window open.
Since “sisters” is the closest subject to the verb and the word is plural, the plural verb “were” is correct. If the singular “brother” was closest to the verb, then “was” is correct, and the sentence would read: “Either her sisters or her brother was responsible for leaving the window open.”
Remember The Proximity Rule to make compound subjects joined by “or/nor” agree with their verbs.