Only actions that have duration can be in the present. I am reading the book. I will be reading the book. When a Russian wants to focus on point C in the reading process, that is, the completion of the action, he will use the perfective aspect of a verb. When part B is emphasized, Russian uses the imperfective aspect.
Most Russian verbs come in pairs.
Let us look at some examples. . Yesterday evening I was reading a book. The speaker here describes part B of the act of reading. She says nothing about finishing reading the book. The listener should not assume that the book was not finished.