The principal parts of a verb are the first person singular indicative of the tense-systems occurring in it. These are generally six: the present, future, first aorist, first (or second) perfect active, the perfect middle, and the first (or second) aorist passive. The future middle is given if there is no future active. The second aorist (active or middle) is added if it occurs. Thus:
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].