Questions & Answers about Koskeeko tämä sääntö poneja?
Why is poneja in the partitive case rather than the illative case?
The verb koskea changes its meaning based on the case of its object (its rection). When it means "to apply to" or "to concern", it requires the partitive case (poneja). If you used the illative case here (poneihin), the sentence would bizarrely ask "Is this rule physically touching the ponies?"
Why is the partitive plural of poni formed as poneja?
For many words ending in -i, including loanwords like poni, the stem changes the final -i to an -e- before adding the plural marker -i-. When the partitive ending -a is added, the plural -i- turns into a -j- between two vowels to make pronunciation smoother. This gives us poni -> plural stem ponei- -> poneja.
Could I attach the question clitic -ko to tämä instead of koskee?
Yes, but it shifts the focus of the question. If you asked Tämäkö sääntö koskee poneja?, you would be specifically asking "Is it this rule (and not some other rule) that applies to the ponies?". By attaching -ko to the verb koskea, you are neutrally asking if the action itself happens.
Why doesn't the subject tämä sääntö change case in this question?