Subjects and verbs have to agree with each other in number. It is not acceptable to mix a plural subject with a singular verb or vice versa. Singular subjects must be paired with singular verbs and plural subjects must be paired with plural verb. It is important to be certain that you are selecting the correct word in the sentence as the subject. Whenever you are dealing with sentences that have prepositional phrases, eliminate the phrases before choosing the subject. Sentences that have inverted word order or begin with here or there often have the subject following the verb rather than in front of the verb. Once again look carefully before selecting the subject to agree with the verb.
In addition to these problems, compound subjects, indefinite pronouns, and collective nouns all present possible agreement problems. Compound subjects joined with and are plural while subjects joined by or or nor have a verb that agrees with the subject closest to the verb. Indefinite pronouns are always singular and collective pronouns are usually singular. Collective nouns can be plural when all the members of the collective unit are functioning as individuals rather than as one unit.