Verbs express a particular action (throw) or state of being (was). In addition, verbs help express who or what performs the action (person), how many people or things perform the action (number), the speaker's attitude toward or relation to the action (mood), and whether the subject is the giver or receiver of the action (voice). Perhaps the most obvious characteristic of verbs, however, is how the verb tenses express time.
Different verb forms are used in combination to express when actions occur. The simple present, past, and future tenses simply place events in time. The perfect tenses (they occur with have, has, and had) express events or actions completed; the progressive tenses (-ing verbs used together with helpers such as is, was, and were) show actions or events that are continuing.