Learn Polish: Nie zamykaj okna, proszę. - Do not close the window, please.

In Polish, negative commands (telling someone not to do something) almost always use the imperfective aspect, even for a single, specific action. If you were telling them TO close the window, you would use the perfective command zamknij.

Why is the word for window okna here, rather than its dictionary form okno?

The verb zamykać usually takes a direct object in the accusative case, which for the neuter noun okno is just okno. However, because the verb is negated (nie zamykaj), the object must change to the genitive case, which is okna. This rule is called the genitive of negation.

How do we get the command form zamykaj from the infinitive zamykać?

For many regular verbs ending in -ać, you can form the informal command for "you" by taking the dictionary form, dropping the final -ć, and adding a -j. So zamykać becomes zamykaj.