Learn Polish: Nie rozlej mleka! - Do not spill the milk!

The base verb is lać, which means "to pour". The prefix roz- indicates dispersal, separation, or spreading things out in multiple directions. When you unintentionally "pour" a liquid so that it spreads everywhere, it means "to spill" (rozlać).

Why is the word for milk in the form mleka instead of mleko?

This is due to the genitive of negation. A positive sentence would use the accusative case for the direct object (Rozlałeś mleko! - You spilled the milk!). However, whenever a direct object is negated with nie, it must switch to the genitive case. The genitive form of the neuter noun mleko is mleka.

How is the command form rozlej constructed?

Rozlej is the informal imperative (command) for "you". Even though the infinitive is rozlać, it conjugates like its base verb lać (I pour = leję, you pour = lejesz). To form the imperative, you take the "you" form of the future perfective (rozlejesz) and drop the -esz ending, leaving the stem rozlej.

Why is a perfective verb (rozlać) used in a negative command here?