Aspect in the imperative

You already know how to build a command (stavi, stavite) and how to forbid something (ne stavljaj, nemoj da stavljaš). You also know that most verbs come in pairs: a perfective one for a finished, single whole (staviti) and an imperfective one for an action seen as ongoing or repeated (stavljati). In a command you have to pick one of the two every time, and the choice says something about the kind of instruction you are giving.

One act, here and now: perfective

When you ask for one concrete thing to be done once and finished, use the perfective: Stavi lonac na šporet. (Put the pot on the stove.) Dodaj mi so. (Pass me the salt.) Uključi rernu. (Turn the oven on.) This is the ordinary, default command.

Again and again, or as a rule: imperfective

Switch to the imperfective when the instruction is not about one act but about a habit or a standing rule:

• repeated action — something to do every time, regularly, from time to time: Svaki put kad kuvaš testeninu, stavljaj malo soli u vodu.
• general instruction or advice — how one should always behave: Uvek perite ruke pre nego što počnete da kuvate.
• an action that should go on for a while — stirring, cooking, frying for some minutes: Mešajte sos stalno.

Forbidding: negation flips the default

This is the part that catches learners. When you tell someone not to do something, use the imperfective, even where the positive command would be perfective. You say Otvori rernu (open the oven, perfective), but Ne otvaraj rernu! (don't open the oven, imperfective). The same holds for every way of forbidding: ne + imperative, nemoj da + present, and nemoj + infinitive.

Do not forbid with ne + a perfective imperative: Ne otvori rernu is wrong. You will hear nemoj da + perfective (Nemoj da staviš previše soli), but it means "be careful not to" — a warning against a slip, not a ban. That is the one exception: a warning about something that could happen by accident — forgetting, dropping, burning: Ne zaboravi so! (Don't forget the salt!) Nobody forgets on purpose, so this is not a ban on an action you would choose to do, and the perfective stays.

A recipe: both aspects in one sequence

In a run of instructions, each step that is done once and then finished is perfective: cut, add, put, pour. A step that has to go on for a stretch of time is imperfective: fry, cook, stir. So a recipe naturally switches back and forth: Iseci luk, stavi ga u tiganj i prži ga pet minuta.