не делай / не сделай

Understanding the split between these two forms is vital for natural Russian communication:

1. The Imperfective (Не + Imperfective Imperative):
This is the most common form. It is used to tell someone to stop an action, to forbid a behavior, or to give general advice. It is also used for polite invitations and welcoming offers (e.g., Не стесняйтесь - Don't be shy). It focuses on the action itself.

• Example: Не открывай окно! (Don't open the window! — I am forbidding you).

2. The Perfective (Не + Perfective Imperative):
This is used as a warning against an unintentional result or an action outside the listener's active control (like a lapse in memory/attention). It is frequently used with verbs of falling, dropping, getting sick, forgetting, or breaking. Native speakers often signal this with the marker Смотри / Смотрите (Watch out). (Note: Постарайся / Попробуй не also express a warning, but they take an infinitive, which can be perfective or imperfective depending on the context, e.g., Постарайся не упасть for a one-time accidental result vs. Постарайся не опаздывать for repeated behavior). The most common perfective negative imperative learners will encounter is Не забудь / Не забудьте (Don't forget).

• Example: Смотри, не разбей окно! (Watch out, don't accidentally break the window!).

3. Verbs of Emotion and State:
Verbs expressing emotions or states (e.g., Не волнуйся, Не бойся, Не плачь) almost exclusively use the imperfective in the negative because they are continuous processes.

4. The Single-Action Rule and Advice:
Even for one-time, specific actions, standard prohibitions and polite requests not to do something use the imperfective. (Note: Strong warnings like Не вздумай or Не посмей are exceptions because they are perfective imperatives used for deliberate prohibitions, though the standard imperfective Не смей! is also very common). To give advice or say something isn't necessary, use Не надо or Не стоит. These words strictly require the imperfective infinitive. Additionally, Нельзя changes meaning based on aspect: Нельзя + imperfective means prohibition (You must not), while Нельзя + perfective means impossibility (It is impossible to).

• Example: Пожалуйста, не закрывайте дверь. (Please don't close the door - polite request).
• Example: Не надо закрывать дверь. (There is no need to close the door / You shouldn't close the door - advice/lack of necessity).

5. The 'Flip' Rule:
Learners often make the mistake of keeping the aspect the same when turning a perfective positive command into a negative prohibition. The logic behind this flip is that a positive command focuses on achieving a specific result (perfective), while a prohibition focuses on avoiding the action entirely (imperfective).
• Positive: Купи молоко! (Buy milk! - Perfective)
• Negative Prohibition: Не покупай молоко! (Don't buy milk! - Imperfective)
• Negative Warning: Смотри, не купи кислое молоко! (Be careful not to buy sour milk! - Perfective)