Questions & Answers about Taikuri tekee uuden tempun, mikä on yllätys.
Why do we use mikä here instead of the relative pronoun joka?
In Finnish, joka refers to the specific noun right before it. If you used joka here, it would mean the trick itself is a surprise. We use mikä when referring back to the entire previous clause — it is the whole fact that the magician is doing a new trick that is the surprise.
Why is the object uuden tempun instead of uusi temppu?
It is the total object of the action. The magician does one complete trick, so the object takes the genitive -n ending. Notice how uusi changes its stem to uude- and temppu drops a p due to consonant gradation to become tempu- before adding the -n.
Could I use the partitive form mitä instead of mikä?
Not in this exact sentence. The relative pronoun needs to act as the subject of the second clause ('which is a surprise'). Because it is the subject of the verb on, it stays in the basic nominative form mikä.
Why doesn't the word yllätys have a partitive ending?