A verb is a word expressing an action or a condition of a subject.
There are three properties which characterize verbs in English--tense, voice, and mood.
In English the fourteen verb tenses express the time or relative time in which an action or condition occurs.
The voice of a verb, passive or active, expresses whether the action is being received by the subject or being done by the subject. The two voices may occur in any tense.
The mood of a verb expresses the conditions under which an action or condition is taking place. In English there are three moods--indicative, subjunctive, or imperative. Indicative and subjunctive can be in any tense; imperative, only in the present tense.
Verbs are also classified according to function. Action verbs show action or possession. Action verbs are either transitive or intransitive. Linking verbs show the condition of the subject. Auxiliary verbs, also called helping verbs, are used with other verbs to change the tense, voice, or condition of the verb.
Conditional verbs are verbs conjugated with could, would, or should to show a possible condition. They may be in any tense.
The principal parts of a verb are the four forms of the verb from which all forms of the verb can be made. In English the four principal parts are the present (or infinitive), the past tense, the past participle, and the present participle. Since the present participle is always formed the same way (add -ing), some lists of principal parts omit it.
For more on most of these forms, see the specific entries in the glossary.