Learn Russian: Смотри, не пролей горячий суп на ковёр. - Watch out, don't spill the hot soup on the carpet.

You are right that direct negative commands ('don't do that!') almost always take the imperfective aspect. However, this sentence is a warning about an accidental action. When you warn someone to be careful not to do something by accident (like spilling soup, falling, or forgetting), you must use не + the perfective aspect.

The current lesson is about using 'смотри, не' + Perfective Future. But isn't пролей an imperative verb?

Excellent catch! Yes, пролей is the perfective imperative. When warning someone with смотри, не ('watch out, don't...'), Russian allows you to use either the perfective future (like прольёшь) or the perfective imperative (like пролей). Both are completely natural, interchangeable ways to warn someone about a potential accident.

What exactly is the role of смотри here?

While смотри literally means 'look' or 'watch' (the imperative form of смотреть), here it isn't about literally looking at something. It acts as a set warning particle meaning 'watch out,' 'be careful,' or 'make sure you don't.'

Why do горячий суп and ковёр look like they are in the nominative (dictionary) case?