Learn Finnish: Mikä on peilin takana? - What is behind the mirror?

Because the word takana (behind) is a postposition. In Finnish, postpositions almost always require the noun they are linked to to be in the genitive case. So, we add the -n ending to peili (mirror) to get peilin takana.

Why does takana come after the word for mirror?

English uses prepositions that go before the noun. Finnish mostly uses postpositions that go directly after the noun. So behind the mirror is constructed as mirror's behind, becoming peilin takana.

Why is it mikä and not mitä for what?

We use mikä (the basic nominative form) when asking what is to identify a specific, whole thing acting as the subject. You would use the partitive form mitä for ongoing actions, uncountable things, or feelings, but here we just want to know the identity of the object behind the mirror.

Does the word takana change if we are talking about more than one mirror?

No, postpositions like takana do not change to match the noun. If you wanted to ask what is behind multiple mirrors, only the noun would change to its plural genitive form, making it peilien takana.