Russian Negative Imperative: Perfective or Imperfective Aspect

Did you know that in the majority of cases we use the imperfective verbal aspect in constructions with negative imperative (не читай, не гуляй, не пиши, etc.). However, we use the perfective verbal aspect if it is a warning, when the person speaking is afraid that something negative can happen in the future to the other person.

For example,

Не забудь дома учебник. Don’t forget the textbook at home.

Смотри не опоздай! Be careful and don’t come late.

Там скользко. Смотри не упади! It is slippery outside. Be careful and don’t fall down.