Learn Finnish: Kansallispuisto on täydellinen kohde niin turisteille kuin paikallisillekin. - The national park is a perfect destination for both tourists and locals.

It translates to "both... and..." or "as well as...". It frames the two groups being compared: niin comes before the first group (turisteille), kuin precedes the second group (paikallisille), and the clitic -kin (meaning "also" or "too") is attached to the very end of the second item to complete the structure.

Why are both turisteille and paikallisille in the allative (-lle) case?

In Finnish, being a destination "for" someone is expressed using the allative case (-lle). Because the niin ... kuin ... -kin structure links the tourists and locals as equal targets of the phrase täydellinen kohde (perfect destination), both nouns must be in the same case.

What happens to the word paikallinen to make it paikallisillekin?

Words ending in -nen, like paikallinen (local), change their stem to -si- in the plural before adding case endings. The plural stem paikallisi- takes the allative ending -lle, resulting in paikallisille (for the locals). Finally, the -kin clitic is added to the very end of the fully inflected word.

Could I use sekä ... että instead of niin ... kuin ... -kin in this sentence?