It has no ongoing-now reading. It names a whole event inside a frame: after kadкад or akoако, or as one step in a sequence.
Secondary imperfective
Often the perfective is the base and the imperfective is derived from it by a suffix: otvoritiотворити → otvaratiотварати. The arrow shows meaning, not history.
na-на- is not only completion
The prefix can also mean onto, up, or enough. Learn the pair as a word first; the prefix is a hint.
po-по-: once, completion
The prefix is broad. It usually bounds the action: take a look, eat up, drink up. The exact feel depends on the verb.
u-у-: into, result
Into a space, or through to a result: ućiући is to go in, uraditiурадити to get it done.
iz-из-: out, fully
Out of something, or all the way: izvaditiизвадити is to take out, izgraditiизградити to build to the end.
do-до-: reach
Up to a point: doćiдоћи is to arrive. Like every prefixed motion verb it is perfective; its imperfective is derived anew (dolazitiдолазити), and the bare verb ićiићи is not its pair.
pro-про-: through, for a span
Through something, or over a stretch of time: pročitatiпрочитати is to read through, provestiпровести to spend (time).