A verb must agree in number with its subject: singular subjects take singular verbs, and plural subjects take plural verbs. Agreement errors arise most often when the subject and verb are separated by intervening phrases, when compound subjects are joined by or/nor, or with collective nouns and indefinite pronouns.
Practice with sentences containing prepositional phrases between subject and verb (The box of chocolates is/are open), forcing learners to look past the intervening noun. Study indefinite pronoun rules (everyone is, few are) as a special case.
Subject-verb agreement is one of the most fundamental rules in English grammar: a singular subject takes a singular verb, and a plural subject takes a plural verb. In simple sentences this is automatic — nobody says "The dog bark" or "The dogs barks." The difficulty arises in longer, more complex sentences where the subject and verb are separated, or where the subject itself has an unusual form.
The most common source of errors is intervening phrases. In "The box of chocolates is on the table," the subject is "box" (singular), not "chocolates." The phrase "of chocolates" is a prepositional phrase that modifies "box" — it describes which box — but it does not change what the subject is. Because the plural noun "chocolates" sits right next to the verb, it is easy to match the verb to it by mistake. The fix is straightforward: mentally cross out the prepositional phrase and ask what remains. "The box... is." Singular subject, singular verb.
A second complication arises with compound subjects joined by or/nor. "Either the captain or the players are responsible" might feel wrong because we expect subject-verb agreement to depend on the whole compound. But the rule for or/nor is proximity: the verb agrees with the subject closest to it. "Either the captain or the players are" (players is closest, plural). "Either the players or the captain is" (captain is closest, singular). This is called the proximity rule.
Collective nouns present a different puzzle. Words like "team," "committee," "jury," and "class" name a group as a unit. In American English, they are nearly always treated as singular: "The team is practicing." The test is whether the group is acting as one unit or as individual members. "The jury has reached a verdict" (one unified action) vs. "The jury were divided in their opinions" (members acting individually, more common in British English). When in doubt, American usage defaults to singular for collective nouns.
Finally, many indefinite pronouns follow fixed agreement rules that must be learned. "Everyone," "someone," "nobody," "each," and "either" are always singular, even though they may seem to refer to multiple people: "Everyone is required to submit their form." "Few," "many," "both," and "several" are always plural. Knowing these prevents a reliable category of agreement errors that grammar checkers often miss.