Learn Polish: Musisz dolać wrzątku do tego dzbanka. - You must add boiling water to this jug.

This is the Partitive Genitive. When using verbs of adding or accumulating, especially those starting with the prefix do- like dolać (to pour more), the direct object takes the Genitive case to mean "some of" that substance.

How does the prefix do- change the meaning of the verb lać here?

The base verb lać means "to pour." Adding the prefix do- makes it perfective and adds the specific meaning of "to pour more of something" or "to top up."

What is the dictionary form of wrzątku and why does the spelling change so much?

The nominative form is wrzątek (boiling water). It contains a "mobile e" (or fleeting e) in the final syllable. When we add the Genitive ending -u, that e is dropped, resulting in wrzątku.

Could I just use the general verb dodać (to add) instead of dolać?

You could, and you would be understood, but Polish strongly prefers specific verbs based on the physical action. Because water is a liquid, you use dolać (from pouring). If you were adding a dry powder like sugar, you would use dosypać.

We also use the Genitive at the end of the sentence: . Is this also the Partitive Genitive?