Questions & Answers about Осторожно, не бросай мяч в окно!
Осторожно literally means carefully / cautiously, but very often it’s used like a standalone warning: Careful! / Watch out!
In this sentence it functions almost like an interjection (a warning word) introducing the command.
This is an aspect choice in the negative imperative:
- не бросай (imperfective) = don’t throw (in general / don’t do that), often a prohibition or repeated/ongoing warning.
- не брось (perfective) = don’t throw (this one time, right now, don’t let it happen), more momentary and less common in everyday prohibitions.
Both can be possible, but не бросай is the “default” for Don’t do that.