The verb-stem may show numerous modifications in form.
Thus, corresponding to the gradations in sing, sang, sung ( cross35), the verb λείπ-ω
a. When the fundamental stem shows modifications, it is customary for convenience to call its shorter (or shortest) form the verb-stem, and to derive the other forms from it. The student must, however, beware of assuming that the short forms are older than the other forms.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].