The Tense-stems.—The tenses fall into nine classes called tense-systems. Each tense-system has its own separate tense-stem.
| SYSTEMS. | TENSES. | ||
| I. | Present, | including | present and imperfect. |
| II. | Future, | “ | future active and middle. |
| III. | First aorist, | “ | first aorist active and middle. |
| IV. | Second aorist, | “ | second aorist active and middle. |
| V. | First perfect, | “ | first perfect, first pluperfect, and fut. perf., active. |
| VI. | Second perfect, | “ | second perfect and second pluperfect active. |
| VII. | Perfect middle, | “ | perfect and pluperfect middle (pass.), future perfect. |
| VIII. | First passive, | “ | first aorist and first future passive. |
| IX. | Second passive, | “ | second aorist and second future passive. |
The tense-stems are explained in detail in 497-597.
a. Since few verbs have both the first and second form of the same tense (361), most verbs have only six of these nine systems; many verbs do not even have six. Scarcely any verb shows all nine systems.
b. There are also secondary tense-stems for the future passive, the pluperfect, and the future perfect.
c. The tense-stems assume separate forms in the different moods.