1. When a singular noun and a plural noun are combined by ‘OR’, ‘EITHER- OR ‘NEITHER- NOR, the singular noun usually comes first in the sentence and the pronoun must be in the plural
number.
Ex.- Either the manager or his subordinates failed in their duty in sending the official message.
2. When two or more singular nouns are joined by Either-Or, neither-nor, the pronoun is always in the singular
Ex.- Ram or Mohan should invest his money in some business.
Neither Ram nor Shyam confessed his guilt
Either Sita or Kamla forgot to take her prize.