Don't do it! (nie + imperative)

Commands flip aspect under nie

Affirmative commands are usually PERFECTIVE — you want the thing done: Zamknij okno! But tell someone NOT to do something and Polish switches to the IMPERFECTIVE:

do it!don't do it!
Zamknij okno!Nie zamykaj okna!
Kup chleb!Nie kupuj chleba!
Otwórz drzwi!Nie otwieraj drzwi!

A prohibition is about the activity itself, not its completion — so the imperfective (zamykać, kupować, otwierać) takes over.

Do not close the window, please.
Nie zamykaj okna, proszę.
Do not buy this cheese, it is old.
Nie kupuj tego sera, jest stary.

Note the object: negation sends the Accusative to the Genitive, as always (okna, sera).

With niech

Third-person commands negate the same way:

Let the children not watch this film.
Niech dzieci nie oglądają tego filmu.

The perfective warning

A negated PERFECTIVE imperative is not a prohibition but a warning about a one-off slip: Nie zgub kluczy! (don't lose the keys!), Nie zapomnij! (don't forget — mind you remember!). Compare Do not forget about our meeting.

Nie zapominaj o naszym spotkaniu. — a general "don't keep forgetting".

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