The subject of a verb, which is always either a noun or a noun phrase, governs whether the verb is singular or plural. If the subject is singular, the verb must be singular as well. In the sentence, "The girl reads," "girl" is the subject and is singular; therefore "reads," the verb, is also singular. In "The girls read," "girls" is the subject and is plural; therefore "read" is also plural. Note that the plural noun and the singular verb take an "s," while a singular noun and a plural verb do not take an "s." However, this is not always the case, as in "The people walk," where the subject "people" is already plural and does not take an "s." This is because nouns like "people," "money" and "water" are non-countable, meaning you can't add say "three moneys." Adding an "s" is necessary only in the present tense, and only with regular verbs. An irregular verb such as "to be" is "are" in the plural and "am" and "is" in the singular.
English contains six different verb tenses, two each for the past, present and future. Additionally, each of the six tenses can take a progressive form. The simple tenses include the simple present, "I talk," the simple past, "I talked" and the simple future, "I will talk." The perfect tenses include the present perfect, "I have talked," the past perfect, "I had talked" and the future perfect, "I will have talked." Each of these forms takes the progressive by adding the verb "to be" and "ing," as in "I was talking." Only the auxiliary, or helper verbs, in these verb phrases take singular or plural forms. In the sentence, "I was talking," "I" is singular and takes "was," an auxiliary verb, while in "They were talking" "they" is plural and takes "were" as its auxiliary verb.
Action vs. Stative Verbs
Action verbs such as "jump," "walk" or "eat" describe a clear physical action. Stative verbs show a state of existence or inert perception, such as the verb "to be" in "I am tired" and "adore" in "I adore him." Action and stative verbs must agree with their subjects and take an "s" or an irregular plural form when the subject is plural.
Sometimes verbs take the forms of nouns or adjectives, and do not function like verbs, although many people mistake them for verbs. Gerunds, such as "swimming" in "Swimming is fun," end in "ing" and function as nouns. Verbs can also act like adjectives, as in, "Boiled water is usually safe to drink," where "boiled" describes "water" and is, therefore, not a verb. Sometimes gerunds can take a plural form, for example, "These readings are difficult."