Tense and mood
There have been many treatments of tense and mood, but two main trends can be discerned. On one side are analysts who see a category as a constellation of distinct uses and who try to itemize all of them. The lists vary according to the listmaker. For the Spanish present indicative, the RAE (1924, 289) found three uses, Michalson and Aires (1981, 47) four, Sallese and Fernández de la Vega (1968, 118) five, and Garcés (1997) six. DaSilva and Lovett (1965, 4) gave four uses for the imperfect and two for the preterite, while the RAE (1979, 466–69) perceived five and three, respectively.
On the other side is a long line of scholars—Bello ("1847" 1958), Ramsey ("1894" 1956), Bull (1965), Gili Gaya (1973), and most since them—who have sought a unified description of each category, that is, one basic, general concept underlying the surface uses. This concept is defined on the basis of how the category contrasts with others in the same system, because the meaning of 'present' in a two-tense system may cover more ground than in a five-tense one. Thus, in order to understand such theories, it is first necessary to examine the overall system.
One of the most coherent theories of tense is that of Bull (1965, 149–71), who distinguished SYSTEMIC from NONSYSTEMIC meaning. The former is the meaning each category has within the overall system, and in general Spanish and English concur here. The latter includes specialized functions it has acquired outside that system by way of extension, and the two languages diverge more in this case.
Bull first noted that the tense one chooses for portraying a situation depends on his/her orientation or perspective: 'right now', for example, vs. 'back then'. Each such orientation is an axis or timeline along which the event being described is located in one of three ways: as ANTERIOR to the orientation the speaker adopts, as SIMULTANEOUS with it, or as POSTERIOR to it. The different tenses in the system