Some general guidelines for making your verb match your subject:
1. The following pronouns take a singular verb:
(However, when "each" follows a plural subject, the verb is plural: "The students each have different ways of studying.")
2. A compound subject preceded by the word "each" or "every" is singular.
3. The following indefinite pronouns are either singular or plural, depending on context of meaning:
4. these indefinite pronouns are plural:
5. the word "number" is singular when preceded by "the" and plural when preceded by "a."
6. A compound subject joined by "and" takes a plural verb.
7. A compound subject joined by anything other than "and," such as "as well as," "in addition to," or "along with" takes a singular verb (if the main subject is singular).
8. A compound subject joined by "or" or "nor" takes a verb that agrees with the subject closest to the verb.
Paper or envelopes were in the desk.
Envelopes or paper was on the shelf. (Yes, that looks wrong, but it isn't)
"either/or" and "neither/nor" follow this same pattern.