Learn Finnish: Miksi te juoksette näyttelyyn? - Why are you running to the exhibition?

Those special endings are for words that already end in a long vowel or a vowel combination (like työ -> työhön or Lontoo -> Lontooseen). The dictionary form here is näyttely (exhibition), which ends in a single short vowel. For these, the illative ("into") case is simpler: just double that final vowel and add -n. So, näyttely + y + n = näyttelyyn.

Where does the -kse- in juoksette come from?

The dictionary form is juosta (to run). This is a "Type 3" verb, which often ends in -sta or -stä. To conjugate it, you drop the -sta and replace it with -kse- before adding your personal ending. Here, we add the plural "you" ending -tte, giving us juoksette.

How do we know it means "are running" instead of just "run"?

Finnish doesn't have a separate "-ing" tense for actions happening right now. Te juoksette translates to both "you run" (in general) and "you are running" (currently). The context of asking "why" someone is moving toward a destination makes "are running" the most natural English translation here.

What would happen if I said näyttelyssä instead of näyttelyyn?