Learn Polish: Niech oni polepszą tę sytuację! - Let them improve this situation!

It is built from lepszy (better), the comparative form of dobry (good). By adding the prefix po- to this comparative root, we create the perfective verb polepszyć (to make better, or to improve). This is a classic example of a factitive verb, showing a change of state.

Why does this sentence use niech instead of a regular imperative command?

Polish verbs only have dedicated imperative endings for you, we, and you all. To give a command to a third party (let him, let her, let them), you use the particle niech followed by the regular present or future tense verb. Here, oni polepszą is the third-person plural future tense.

Is polepszą perfective or imperfective, and how does that affect the meaning here?

It is perfective. When used with niech for a command, a perfective verb like polepszą focuses on the successful completion of the result. You are demanding that they actually get the situation improved, not just that they spend time working on it.

I often hear native speakers say tą sytuację. Why does this sentence use tę sytuację?