Learn Polish: Słuchanie muzyki pomaga na stres. - Listening to music helps with stress.

It comes from the imperfective verb słuchać (to listen). To form it, we drop the final -ć and add the suffix -nie. Like all verbal nouns ending in -nie or -cie, słuchanie is grammatically neuter and declines just like a standard noun.

Why is muzyki in the Genitive case here?

There are two overlapping reasons! First, in Polish, a noun acting as the object of a verbal noun usually takes the Genitive (translating roughly to "the listening of music"). Second, the root verb słuchać already requires the Genitive case anyway (słucham muzyki), and verbal nouns keep the case requirements of the verbs they come from.

In English we say "helps with stress," but here it's pomaga na stres. Why use na?

When talking about a remedy, medicine, or relief for a negative condition (like an illness, pain, or stress), Polish specifically uses the verb pomagać + na. Translating "helps with" literally by using z (with) would sound unnatural here.

What case is stres following the preposition na?

It is in the Accusative case. The phrase pomagać na requires the Accusative for the ailment being treated. Because stres is an inanimate masculine noun, its Accusative form looks exactly the same as its basic dictionary (Nominative) form.