1. The Imperfect is, to put it simple, a Past opposed to the Present.
2. There are some possibilities for the opposition of Present and Imperfect stems:
a. The Present stem can be a reduplicated root and the Imperfect a basic root; as, sista-mi,/ sta-m.
b. The Present is a thematic root, while the Imperfect is athematic in -s; as, leiq-o, leiq-s-m.
c. Both are thematic roots, but with distinct vowel degrees; as, leq-o, liq-om.
NOTE. Every stem can usually be Present or Imperfect, provided that they are opposed to each other. And there can be more than one Present and Imperfect from the same Root.
3. There is a logical trend to specialize the roles, so that those Stems which are rare in Present are usual in Imperfects. For example, the thematic roots, as well as those in -s-. There is, in fact, only one real confusion problem when distinguishing stems, and that is when they end in -e or -a, as they are given in Presents and Imperfects alike. It is by way of oppositions and formal specializations that they can be distinguished.