Learn Finnish: Eräs noppa on pöydän alla. - A certain die is under the table.

Eräs means "a certain" or "one particular". While yksi just means the number one, and joku means "some" or "any" person or thing, eräs implies that a specific die is being talked about, even if it is not explicitly identified to the listener. It belongs to the same broad family of indefinite pronouns as the lesson's focus, kukin.

Why does pöytä become pöydän in this sentence?

The word alla (under) is a postposition. Most Finnish postpositions require the noun before them to be in the genitive case. So, pöytä gets the genitive -n ending. Notice also the consonant gradation: the strong t in pöytä changes to a weak d in pöydän.

How does alla differ from alle and alta?

Finnish postpositions change their endings depending on movement, just like local cases. Alla means something is statically "under" or "beneath" something else (location). If the die were rolling to under the table, you would use alle, and if you were picking it up from under the table, you would use alta.

Does noppa mean a single "die" or plural "dice"?