It’s ‘All about –μι’:

What we call ‘the –μι verbs’ should not be as troublesome as they may seem.

The thing to remember is that these verbs are athematic in the present (and aorist as well)—so there is no thematic vowel, o/e, to complicate things. 

We’ve already learned how to do the athematic middle (=passive) forms:

that was the pattern we learned in ch. 16 for δύναμαι, κείμαι, ἐπίσταμαι; you just add the endings directly to that vowel stem, δυνα + σαι, + ται, etc.

I.       For the middle/passive forms of –μι verbs, you do the same thing (using the short vowel version of the stem): p. 22

The imperfect works the same way:

II. It’s in the present active that things get interesting—in two ways:

a) there is an alternation between long-vowel and short-vowel bases:

long-vowel for singulars; short-vowel for plurals, infinitive and participle.

b) and the endings show a small variation for ‘I’ and ‘s/he’ and ‘they’:

The participles take that same short-vowel stem

A similar alternation is seen in the aorist (strong-form singular/short-vowel plural)

and stem drops reduplication:

 The middle/passive, again, uses the short-vowel version:

ἐ-δό-μην, ἐδο-σο --> ἔδου, ἔδοτο, κτλ.

ἐ-θέ-μην, ἐθε-σο --> ἔθου, ἔθετο, κτλ.