Learn Finnish: Onkohan hämähäkinverkon takana kummitus? - I wonder if there is a ghost behind the spiderweb?

It combines the question clitic -ko and the softening clitic -han. Adding -kohan to the verb on (is) changes it from a direct question (onko, meaning 'is there?') into a pondering one ('I wonder if there is?').

Why does hämähäkinverkon end in -n?

It is in the genitive case because of the postposition takana (behind). In Finnish, postpositions almost always require the noun right before them to take the genitive -n ending. You can think of it literally as 'at the back of the spiderweb'.

Hämähäkinverkon is a long word! How is it put together?

It is a compound word. The first part is hämähäkki (spider) in the genitive form (hämähäkin), making it 'spider's'. The second part is verkko (web or net). Then the entire compound word takes a final -n to match the postposition takana.

Why is the subject kummitus (ghost) placed at the very end of the sentence?

This is an existential sentence, which states that something exists in a location. In regular statements, these follow a Location + Verb + Subject order. When turned into a question, the verb (onkohan) moves to the front, the location (hämähäkinverkon takana) follows, and the subject (kummitus) stays comfortably at the end.