Herbert Weir Smyth, A Greek Grammar for Colleges

a. Labial Stems.—λέλειμ-μαι is for λελειπ-μαι, λέλειφ-θον is for λελειπ-σθον, λέλειφθε is for λελειπ-σθε (103). In the same manner are inflected other labial stems, as τρί_βω (τρι_β-) rub, ῥί_πτω (ῥι_π-) throw: τέτρι_μ-μαι for τετρι_β-μαι, τέτρι_ψαι for τετρι_β-σαι, etc. Stems ending in μπ drop π before μ, but retain it before other consonants. Thus,

b. Dental Stems.—πέπεισ-ται is for πεπειθ-ται (83), πέπεισ-θον is for πεπειθθον (83), πέπεισθε is for πεπειθ-（ς）θε (83, 103). The ς thus produced was trans ferred to the first persons πέπεισμαι, πεπείσμεθα (86, 87). Like πέπεισμαι, etc., are formed and inflected ἔψευσμαι from ψεύδω (ψευδ-) deceive, πέφρασμαι from φράζω (φραδ-) declare, ἔσπεισμαι (100) from σπένδω (σπενδ-) pour a libation.