Primary and Secondary Tenses.—The primary tenses refer to present and future time (present, future, perfect, and future perfect), the secondary or historical tenses refer to past time (imperfect, aorist, pluperfect).
a. The gnomic aorist ( cross1931 b) is regarded as a primary tense, as is the aorist when used for the perfect ( cross1940), and the imperfect indicative referring to present time ( cross1788); the historical present ( cross1883), as a secondary tense. The subjunctive, optative, and imperative moods in their independent uses point to the future, and all their tenses therefore count as primary.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].