In the Greek language, there were two basic ways verbs were spelled to show their person, gender, number, etc. In the NT, the omega conjugation is by far the most popular. The μι-verb conjugation is slowly dying off. There are still, however, some μι verbs in the NT; e.g. ιστημι, ειμι, διδωμι, etc. Study BBG chapter 34 and 35 or this video.
IV. The Survival of μι-verbs.
(a) A CROSS DIVISION. Before we take up modes, voices, tenses, we are confronted with a double method of inflection that cuts across the modes, voices and tenses. One is called the -μι inflection from the immediate attachment of the personal endings to the stem. The other is the -ω inflection and has the thematic vowel added to the stem. But the difference of inflection is not general throughout any verb, only in the second aorist and the present-tense systems (and a few second perfects), and even so the -μι conjugation is confined to four very common verbs (ιημι, ιστημι, διδωμι, τιθημι), except that a number have it either in the present system, like δεικνυμι (with nu inserted here), or the aorist, like εβην. …
(b) THE OLDEST VERBS. This fact is a commonplace in Greek grammar. It is probable that originally all verbs were -μι verbs. This inflection is preserved in optative forms like λυοιμι, and in Homer the subjunctive εθελωμι, ιδωμι, etc. The simplest roots with the most elementary ideas have the -μι form. Hence the conclusion is obvious that the -μι conjugation that survives in some verbs in the second aorist and present systems (one or both) is the original. It was in the beginning λεγομι with thematic as well as φημι with non-thematic verbs. source