The Subjunctive is normally athematic, usually in -a, -e and sometimes -o, always opposed to the Indicatives. There are also Subjunctives in -s, probably newer than those in -e, -a.
NOTE. It is a known archaism (for a few scholars an innovation) of the Baltoslavic that it doesn't have a subjunctive which would be then an innovation of IE III, not given in IE II.
There are, thus, two possibilities when forming the Subjunctive:
a. Indicative thematic / Subjunctive with lengthened vowel; as, bhere- / bhere-, deike- / deike-, etc.
b. Indicative athematic / Subjunctive thematic.