It's not about whether the action had begun or not, it's about finishing it. Perfective forms imply that you're going to finish the action in the time you're referring to, no matter if you'd started it earlier or you're about to start now. So przeczytaj książkę (dzisiaj) means: the book is supposed to be finished (today), czytaj książkę says: go read as long as you want (it's not said you're ever supposed to finish it, however this is logical). It goes the same with maluj/namaluj obraz, ucz się/naucz się, sprzątaj/posprzątaj and many many more. In reality we usually use imperfective forms when we're either hurrying somebody or we know that the process in long and it needs more days. It can also mean you're irritated usiądź - normal, siadaj - a bit impolite zjedz - normal, usually before the action begins, jedz - hurrying somebody naucz się - do it today, ucz się - do it cause you still have a lot to do (not impolite in this case)