Learn Finnish: Onko kirjastossa hyvä kirja? - Is there a good book in the library?

Finnish doesn't use a filler word like "there" to say "there is" or "is there". Instead, existential sentences just use the verb olla (to be) alongside a location. Adding the question ending -ko to on (is) creates Onko, which translates perfectly to "Is there...?" in this context.

The lesson says existential sentences start with the location. Why does this one start with the verb?

In a regular statement, the location does come first: Kirjastossa on hyvä kirja (There is a good book in the library). However, to form a yes/no question in Finnish, the word with the question particle -ko or -kö always jumps to the very front of the sentence. So, Onko moves to the beginning, pushing the location (kirjastossa) to second place.

What does the -ssa ending on kirjastossa mean?

The -ssa ending means "in" or "inside" (this is called the inessive case). It attaches to the basic word kirjasto (library) to create the location part of our existential sentence: "in the library".

Why are hyvä and kirja in their basic dictionary forms here?

In existential sentences, the thing that exists (the subject) is usually in its basic form (the nominative case) if it is a whole, countable object. Because a book is a single, countable thing, it stays as hyvä kirja. If it were an uncountable substance like water or coffee, it would need a different ending.