1. If both subjects are singular, the verb which follows "either... or" must be in the singular.
e.g.
a. Either his father or his mother is ill.
b. Neither the boy nor the girl speaks English.
2 If both subjects are plural, then the conjunctions "either... or", "neither ... nor" must be followed by a plural verbs.
e.g.
a. Either my friends or your friends have done it.
b. Neither his brothers nor his sisters are good.
3. If one subject is singular and the other plural, the verb is in the plural.
e.g.
a. Either he or they have it.
b. Neither she nor we are angry.
4. If one subject is plural and the other singular, the verb is in the singular.
e.g.
a. Either they or he has it.
b. Neither we nor she is angry.