Hom. has βούλεαι, perf. μέμνηαι, but pres. δύνασαι, παρίστασαι; ὄψει is unique (for ὄψεαι); subj. δύνηαι. Doric often contracts, as οἴῃ for οἴε-αι. Aeolic generally leaves εαι open (κείσε-αι). Hdt. has open -εαι, -ηαι.
b. 2 Sing.—-σο stays in all plups. and in the imperf. of μι-verbs. Elsewhere it loses its ς, as in ἐλύ_ου from ἐλύ_ε-σο, ἐλύ_σω from ἐλύ_σα-σο, ἐφήνω from ἐφήνα-σο, ἐλίπου from ἐλίπε-σο, ἔθου from ἔθε-σο, ἐπρίω from ἐπρία-σο, ἐτι_μῶ from ἐτι_μάε-σο, ἐφιλοῦ from ἐφιλέε-σο. In the optative, λύ_οιο, λίποιο, τιθεῖο, εἷο, λύ_σαιο, from λύ_οι-σο, etc.; τι_μῷο from τι_μάοι-σο.
N. 1.—ἐδύνω or ἠδύνω and ἠπίστω are commoner than ἐδύνασο and ἠπίστασο from δύναμαι
N. 2.—After a diphthong or a long vowel in the 2 aor. indic. mid. -σο is retained, as εἷσο (ἵ_ημι
b. Hom., Doric, and Aeolic have generally open forms, as Hom. βάλλε-ο (rarely βάλλευ), ὠδύσα-ο. ἔρειο, σπεῖο are from -εεο. Hom. has ἐμάρναο for Attic ἐμάρνασο, and may drop ς even in the pluperfect (ἔσσυο). When Doric contracts αο we have α_. In Hdt. αο, εο are open, but the writing ευ for εο is found.
c. Dual.—The 1 pl. is used for the 1 dual except in the three poetic forms περιδώμεθον, λελείμμεθον, ὁρμώμεθον. Hom. has -σθον for -σθην in θωρήσσεσθον.
d. 1 Pl.—In epic and dramatic poetry -μεσθα is often used for -μεθα for metrical reasons (βουλόμεσθα, ἐπιστάμεσθα).
e. 2 Pl.—On the loss of ς in σθε (ἔσταλθε), see cross103.
f. 3 Pl.—After vowel stems -νται, -ντο are preserved. After stems ending in a consonant -νται, -ντο became -αται, -ατο by 35 b. These forms were retained in prose till about 400 B.C. (e.g. τετάχαται, ἐτετάχατο).
f. -αται, -ατο occur in Hom. regularly in the perfect and pluperfect of consonant stems, as τετράφαται, ἕαται for ἑς-νται, ἥατο for ἡς-ντο from ἧμαι (ἡσμαι); also in stems ending in -ι, as ἐφθίατο. -αται, -ατο were transferred to vocalic stems, as βεβλήαται, βεβλήατο, Hdt. δυνέαται. Hom. has -δ-αται in ἐληλάδαται from ἐλαύνω
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].