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
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].