"Five wise hedgehogs are sitting in the middle of a large rock."
Translation:Viisi viisasta siiliä istuu keskellä suurta kiveä.
The plural they suggest is siivit, but the answer had a partitive plural? Why is this, and does the partitive always take the singular verb (e.g. istuu) instead of the plural (istuvat?)?
I believe your word "siivit" was just a typo (should be "siilit"), so I'm not saying more to that. Then the answer to your question:
Numerals greater than one (1) cause the plurals work in singular. I actually tried to explain this in another exercise a few days ago as you basically can have things three ways only: 1 of them, some of them or all of them. So either 1 hedgehog or all of the hedgehogs in the world, but when you have something between those numbers, it's not the whole thing nor whole group and therefore it takes the PARTitive case. Hope this helps remembering how it goes :) And without the numeral you naturally use the correct verb of the plural, "siilit istuvat" vs. "viisi siiliä istuu".