Learn Finnish: Älä koske poniin kotieläinpihalla. - Do not touch the pony at the petting zoo.

The verb koskea has two different meanings depending on the case it governs. When it means "to physically touch" (like petting a pony), it requires the illative case (poniin). If it meant "to concern" or "to apply to", it would take the partitive case (ponia).

Doesn't a negative command like "Älä" usually force the object to be in the partitive case?

Yes, standard grammatical objects become partitive in negative sentences. However, koskea is a verb with a fixed case rection. It strictly demands the illative case for physical touching, and this specific requirement overrides the general negative partitive rule.

How does the compound word "kotieläinpihalla" break down?

It consists of koti (home) + eläin (animal) + piha (yard/courtyard) + the adessive ending -lla (at). A kotieläin is a domestic animal, so a kotieläinpiha translates to a petting zoo or domestic animal yard.

Why do we use the adessive case (-lla) for the petting zoo instead of the inessive (-ssa)?

The core word here is piha (yard). In Finnish, open outdoor spaces, squares, and yards conventionally take the outer local cases like the adessive (-lla, on/at) rather than the inner local cases (-ssa, in/inside) because you are standing on the yard rather than enclosed within it.