So then (ara oun). Two inferential particles, accordingly therefore, as in 2Th 2:15 and only in Paul in N.T.
Let us not sleep (mē katheudōmen). Present active subjunctive (volitive), let us not go on sleeping.
Let us watch (grēgorōmen). Present active subj. (volitive) again, let us keep awake (late verb grēgoreō from perfect egrēgora).
Be sober (nēphōmen). Present active subjunctive (volitive). Old verb not to be drunk. In N.T. only in figurative sense, to be calm, sober-minded. Also in 1Th 5:8 with the metaphor of drunkenness in contrast.