Learn Finnish: Mikään ei tuhoa kansallispuiston koskematonta luontoa. - Nothing destroys the pristine nature in the national park.

Because the sentence is negative (ei tuhoa). In Finnish, the object of a negative sentence is always in the partitive case, regardless of whether the action is complete or not.

How is the adjective koskematonta (pristine/untouched) formed?

The base form is koskematon. It is created by taking the verb koskea (to touch) and adding the negative suffix -maton or -mätön, which translates to "un-" or "-less" in English. Here it takes the partitive ending -ta to agree with luontoa.

Finnish has no single word for "nothing." How is that expressed here?

Finnish uses a double negative structure. The pronoun mikään (any) must be paired with the negative verb ei to mean "nothing." You cannot omit the negative verb; mikään ei tuhoa literally translates to "anything does not destroy."

What is the base form of the verb tuhoa, and why does it look like this here?

The base form is tuhota (to destroy), which is a Type 4 verb. To form the present negative, you take the infinitive, drop the final -a or -ä, and use the resulting stem. Therefore, tuhota becomes ei tuhoa.