If a present tense verb or helping verb doesn't have the correct form for its subject, you have an error in agreement. For most verbs, this means that the "he" form, or third person singular, takes an "s," as in "He runs." The sentence "He run" therefore has an error in agreement. A singular noun as subject also takes the verb with "s," as in "John runs" and "The water runs." (See Reference 1.)
Other errors arise if you do not match a verb properly with its compound subject. Two or more nouns joined by "and" form a compound subject and require a plural verb as follows: "Mary and John like soccer." The sentence "John and Mary likes soccer" therefore has an error in agreement. Two nouns joined by "or," however, form a singular subject: "John or Mary will drive the car." (See Reference 1.)
Some words cause errors in agreement because they do not look singular. Words ending in "-body" or "-one" are singular, for example "nobody" and "everyone." We write: "Nobody is ready." Also: "Everyone likes Jane." "Either" and "neither" also take the singular: "Neither wrench works." Collective nouns such as "group" or "family" are usually singular, as in "My family is at home." Some nouns that look plural because they end in "s" are also singular, for example "news" and "mathematics." We write: "The news is very bad." (See Reference 1.)
Extra words that separate the subject and verb do not normally change whether the subject is singular or plural. "One of my friends likes to ski." The subject is "one," not friends, so the sentence needs a singular verb: "likes." (See Reference 1.)
Pronouns give rise to other errors in agreement in English. Pronouns must agree with the nouns they refer to, which we call their antecedents. Pronoun agreement in English can require harmony in both gender and number. For example, use "he" to refer to one previously mentioned male, "she" to a female, and "it" to an object. Use "they" to refer back to a plural noun of any gender or mixed genders. "When the girls and boys were ready, they all got on the bus." (See References 2, 3, 4.)
Attempts to avoid naming a particular gender in general statements can cause agreement problems or awkwardness. Traditionally, writers used the masculine gender to refer to males or females, for example: "Each employee should bring his handbook to the meeting." Although some authors now substitute, "his or her," this sounds awkward. Sidestep the problem by using the plural: "Employees should bring their handbooks to the meeting." In informal English, native speakers often use the plural "their" instead of singular "his" or "her" when referring to singular words like "everyone": "We will begin when everyone takes their seats." In formal writing, however, avoid this lack of agreement. (See References 1, 4.)