Verbs are words that express action. Verbs can also express states of being or conditions. In a sentence, verbs have a subject (the doer of the action). Verbs can also have an object (the receiver of the action). Verbs agree in number and person with their subjects, for example: I am, he is, they are.
Verbs express tense; that is, the time at which the action occurred. The most common verb tenses are past, present, and future, but English also has progressive and perfect forms of each of these tenses as well. Sometimes tense is expressed by a change in the form of the verb, and sometimes tense is expressed by “helping verbs” or auxiliary verbs, usually forms of to be, to have, or to do, which supplement the main verb.
Verbs also express voice and mood. The voice can be active (where the subject does the action) or passive (where the action is done by an unspecified or implied subject). Mood can be indicative, imperative (for expressing commands), or subjunctive (for expressing desired, hypothetical, or uncertain events). Modal auxiliaries (must, could, and would) express possible, conditional, or required actions, and are often followed by subjunctive verb forms.
Some verbs require more than one word to express their meaning. These are called phrasal verbs. Finally, some word forms derived from verbs actually have quite different grammatical functions. Gerunds and infinitives act as nouns, while participles act as modifiers.