Perhaps the most basic rule of grammar is that a verb must agree with its subject in number. If the subject is singular, the verb must be singular. If the subject is plural, the verb must also be plural.
A verb should agree in number with its simple subject. Do not be misled by intervening phrases or clauses. In each of the following sentences, the verb and its simple subject have been written in italics.
Singular: The young woman with five excellent recommendations is being considered for the position.
Plural: Two young women with excellent recommendations are being considered for the position.
Singular: Mr. Lowe, as well as his staff members, has been informed. (In determining whether a subject is singular or plural, ignore phrases that begin with expressions such as together with, in addition to, rather than, as well as, along with, including, or accompanied by.)
Plural: The staff members, as well as Mr. Lowe, have been informed. (The simple subject is members; therefore, a plural verb is used.)
The subjects none, some, any, all, most, and fractions, such as half, may be singular or plural. The noun or pronoun to which the subject refers will determine its number. If a prepositional phrase with a plural object follows the subject, the subject will be considered plural. (Exceptions: None may be used to mean not one; any may be used to mean any one.)
Singular: A third of the shipment has been stolen. (The subject refers to shipment, which is singular.)
Plural: A third of the boxes have been stolen. (The subject refers to boxes, which is plural.)
Collective nouns may be singular or plural. If the noun refers to a group as a single unit, a singular verb should be used. If the sentence suggests that members of the group are acting independently of one another, a plural verb should be used.
Singular: The audience is quiet and attentive.
Plural: The audience (or members of the audience) are going their separate ways.
If or or nor is used to join two singular subjects, a singular verb should be used. If a singular subject and a plural subject are joined by or or nor, the plural form should be placed closer to the verb and a plural verb used.
Singular: Neither Marin nor his brother is willing to cooperate.
Plural: Neither Martin nor his brothers are willing to cooperate.
Singular: Bob or Paul is probably responsible.
Plural: Bob or his two associates are probably responsible.