One key convention of the English language is the agreement in number between the subject and verb in a sentence. Both the subject and verb should be singular, or both, plural. [EDIT]
Subject/verb agreement, as with verb tense use, is another primary marker for readers when they react to the work of writers. Several issues cause writers problems. If you have trouble with subject/verb agreement, please consult a resource; I recommend this on-line help:
Note that singular and plural forms for nouns and verbs appear to be opposite. Nouns add an “-s” to form the plural (in regular forms), but verbs are singular with the “-s” and plural without the “-s.”
Another common problem for writers is when the subject and verb are separated by phrases and clauses.
Also be cautious when using collective nouns, when using compound subjects joined by “or,” and when using “there [to be]” constructions. All of these problem areas are addressed in the on-line source noted above.
Finally, be cautious with indefinite pronouns:
Always singular: each, either, neither, one, everyone, no one, nobody, anyone, anybody, someone, somebody, everybody, much
Always plural: several, few, both, many, others.
Either singular or plural, depending on reference: some, any, none, all, most.