Learn Finnish: Ostin uuden puhelimen. Se oli kallis. - I bought a new phone. It was expensive.

They make up the object of the sentence, meaning they are the thing you bought. Because the buying action is complete and you bought one whole phone, Finnish requires the object to take the genitive case ending -n. The adjective uusi (new) must grammatically match the noun it describes, so both words get the ending.

How does the basic word uusi turn into uuden?

Finnish words ending in -si often change to a -te- stem when adding endings, making the base uute-. Then, because adding the -n ending closes the last syllable, the strong consonant t softens to a d through a rule called consonant gradation. This gives us uuden.

What happens to the word puhelin to make it puhelimen?

Finnish nouns ending in -in follow a special pattern. The -in changes to -ime- before you add any case endings. So, the base stem of puhelin becomes puhelime-, and then we add the -n ending.

Why is kallis in its basic form and not the partitive case?