Tense is the grammatical capability of verbs to show by distinctive forms or arrangements the time when the action of the verb occurs, occurred, or will occur. "Tense" is derived from Latin tempus (time). Any set of forms or structures signaling a particular time relation is called a tense of the language (as, present tense, past tense). Among grammatical categories, tense is one of the most complex and controversial because (1) time as such is an abstruse philosophical notion; (2) grammarians cannot agree on a way of analyzing the linguistic markers of time relation; (3) known languages present a bewildering array of time patternings; and (4) tense merges with mood or mode in analyzing verbs. Recognizing the commonsense division of time into "now," "before now," and “after now," traditional grammarians say that a modem English finite verb has six tenses: the present (he walks), the past or preterit (he walked), the future (he will walk), the present perfect (he has walked), the past perfect or pluperfect (he had walked), and the future perfect (he will have walked). They also identify a "progressive tense" (he is walking, was walking) and an "emphatic" conjugation (he does walk, did walk). Structural-descriptive grammarians prefer to restrict the term "tense" to cases in which the form (sound and spelling) of the base verb actually changes to signal a time relation. Hence, for them, modern English has only two tenses the present or common tense and the past or preterit. They deal with the other traditional tenses as phrasal verbs varying in aspect, phase, and mood or mode. Aspect and phase deal not with the now, before now, and after now but with the state of the action-is it beginning, going on, or finished? The durative aspect, for example, signals an action in process (he is walking); the perfect phase signals an action completed (he has walked). The "future tense" employs a modal auxiliary ("shall" or "will"), which is grammatically analogous to "can," "may," "must," and the like. This analysis not only stresses the formal characteristics of English but also recognizes the historical development of the English tense system from the combination of base forms with various auxiliaries.