In grammar, a word that describes an action such as “run” or “think” is a verb.
Verbs are one of the major parts of speech in English. They can express an action, an event, or a state of being: run describes movement, think describes a mental activity, happen describes an event, and exist describes a state. A sentence normally needs a verb or verb phrase to say what is happening or what a subject is like.
Verbs also help show grammatical information. English verbs can appear in forms associated with time, such as walk, walked, and will walk. They can also combine with auxiliary verbs to express meanings such as possibility, obligation, or continuation: may go, must leave, and is reading.
The wording “describes an action” is useful for beginners, but verbs are not limited to physical actions. Words such as know, believe, seem, and be are verbs even though they describe thoughts, relationships, appearances, or states. An adverb modifies a verb or another word, an adjective describes a noun, and a noun generally names a person, place, thing, or idea.