Learn Finnish: Nämä naapurit ärsyyntyvät juhlista. - These neighbors get annoyed by the party.

Verbs that describe an emotional reaction or a change of state, like ärsyyntyä (to get annoyed), require the cause of that emotion to be in the elative case (-sta/-stä). It helps to imagine that the neighbors' annoyance is originating from the party.

The English translation says "the party" (singular). Why is juhlista in the plural form?

In Finnish, the word for a party or festivity is typically used in the plural: juhlat. Because the base word is plural, the elative ending attaches to the plural stem, giving us juhlista rather than the singular juhlasta.

I've seen the verb ärsyttää before. How is ärsyyntyä different?

Ärsyttää means "to annoy" someone else (e.g., the party annoys the neighbors). Ärsyyntyä is its reflexive pair, meaning "to get annoyed" yourself. The -ntyä ending often shows that the subject is experiencing a change of state or emotion without directly acting on an outside object.

Why doesn't nämä have a -t at the end to match the plural naapurit?

Nämä (these) is already the plural form of the pronoun tämä (this). Unlike regular nouns and adjectives that simply add a -t for the nominative plural, these demonstrative pronouns have unique plural forms you just have to memorize.