Liquid Verbs.—Verb-stems ending in λ, μ, ν, ρ lose ς and lengthen their vowel in compensation ( cross37): α to η (after ι or ρ to α_), ε to ει, ι to ι_, υ to υ_.
a. Some verbs in -αινω (-αν-) have -α_να instead of -ηνα; as γλυκαίνω
b. The poetic verbs retaining ς in the future ( cross536) retain it also in the aorist.
c. αἴρω (ἀρ-) raise is treated as if its verb-stem were ἀ_ρ- (contracted from ἀερ- in ἀείρω): aor. ἦρα, ἄ_ρω, ἄ_ραιμι, ἆρον, ἆραι, ἄ_ρας, and ἠράμην, ἄ_ρωμαι, ἀ_ραίμην, ἄ_ρασθαι, ἀ_ράμενος.
d. ἤνεγκα is used as the first aorist of φέρω
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].