Most μι-verbs of the fourth class add -νυ- (after a vowel, -ννυ-) to the verb-stem.729
Verb-stems in -α: κερά-ννυ_μι
Verb-stems in ε (for ες): ἕ-ννυ_μι (in prose ἀμφιέ-ννυ_μι) clothe, κορέννυ_μι
Verb-stems in ω: ζώ-ννυ_μι
All the forms in -ννυ_μι started from verb-stems ending in ς: ἕννυ_μι from ἑς-νυ_-μι, σβέννυ_μι from σβες-νυ_-μι, ζώννυ_μι from ζως-νυ_-μι. All the other verbs are analogues of these.733
Verb-stems in a consonant: ἄγ-νυ_μι
Poetic verbs: αἴ-νυμαι
The verbs whose verb-stem ends in a liquid or nasal often form the tenses other than the present by adding ε or ο, as ὄλλυ_μι (from ὀλνυ_μι) ὤλεσα, ὀλώλεκα (ὀλ-ε-), ὄμνυ_μι ὤμοσα (ὀμ-ο-).736
νυ_μι-verbs form only the present and imperfect according to the μι- conjugation; with the exception of σβέ-ννυ_μι, which has 2 aor. ἔσβην. The 2 aorist passive and 2 future passive are rare, as ῥήγνυ_μι ἐρράγην ἐκραγήσομαι, ζεύγνυ_μι ἐζύγην.736D
From verbs in -νυ_μι second aorists middle are formed in Hom. by only three verbs: μείγνυ_μι (commonly written μί_γνυ_μι)
-νημι class. A few verbs add νη- in the singular, να- in the plural, to the verb-stem. These verbs are almost entirely poetical or dialectical; and show by-forms in -ναω. They are:
a. Only in the middle: μάρναμαι
Stem Gradation.—Verbs of the root class show in the stem vowel a variation between strong and weak grades in the present and imperfect indicative active. The singular has the strong grade, the dual and plural have the weak grade. The optative active and most middle forms have the weak grade.
a. η strong (original and Dor. α_), α weak; φημί φαμέν, ἔφην ἔφαμεν; ἵστημι ἵσταμεν, ἵ_στην ἵ_σταμεν; δάμνημι δάμναμεν.
b. η strong, ε weak: τίθημι τίθεμεν, ἐτίθην ἐτίθεμεν; ἵ_ημι ἵ_εμεν.
c. ω strong, ο weak: δίδωμι δίδομεν.
d. ει strong, ι weak (cp. λείπω ἔλιπον): εἶμι
In the second aorist ἔστην
A few root verbs retain the strong grade η throughout. Thus, poet. ἄημι
Verbs adding νυ show the strong form of the verb-stem in the present. ῥήγ-νυ_-μι
The ending νυ varies between strong νυ_ and weak νυ. Thus δείκνυ_μι δείκνυμεν, ἐδείκνυ_ν ἐδείκνυμεν.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].