Learn Finnish: Onkohan tämän vanhan laatikon takana kätkö? - I wonder if there is a stash behind this old box?

It is made of three parts. It starts with the verb on (is). Then the question particle -ko makes it a yes/no question ('is there?'). Finally, the clitic -han softens the question and adds a sense of pondering. Together, the fused clitic -kohan is the standard way to express 'I wonder if' in Finnish.

Why are the words for 'this old box' in the genitive case (tämän vanhan laatikon)?

They are in the genitive case because they come right before the postposition takana (behind). In Finnish, almost all postpositions require the noun phrase preceding them to take the genitive -n ending.

Why is the subject kätkö (stash) placed at the very end of the sentence?

This is an existential sentence, which states that something exists in a location. In a normal statement, the location comes first and the subject is last. Because this is a question, the verb onkohan is pulled to the front, but the subject kätkö naturally stays in its usual spot at the end.

Can the postposition takana change forms if the action involves movement?