Learn Finnish: Seisooko näyttelijä keskellä lavaa? - Does the actor stand in the middle of the stage?

In this sentence, lavaa comes right after the preposition keskellä (in the middle of). Keskellä is a preposition that requires the noun following it to be in the partitive case, so we add an -a to lava.

I've seen keskellä used after a noun before. Can it be a postposition too?

Yes! Keskellä is flexible. Here it is a preposition taking the partitive case (keskellä lavaa). It can also be a postposition coming after the noun. If you use it as a postposition, the noun needs to be in the genitive case instead: lavan keskellä. Both mean exactly the same thing.

How does Seisooko form the question "Does... stand"?

To make a simple yes/no question in Finnish, you take the main verb—here, seisoo (he/she/it stands)—add the question particle -ko, and put it at the very beginning of the sentence. Finnish doesn't use helper words like "does" or "do" for questions.

I recognize the end of näyttelijä (actor). Is that a common suffix?

Yes, the -ja or -jä suffix is used to turn a verb into a noun for a person who does that action. Here, the verb näytellä (to act) becomes näyttelijä (actor). It is very similar to the "-er" or "-or" endings in English!