Learn Finnish: Hän ei arvostanut nuoruutta. - He did not appreciate youth.

To make an abstract noun out of nuori, we add the suffix -uus. The final vowel i of the adjective's stem is dropped before adding the suffix, giving us nuoruus.

Why does the word nuoruus change to nuoruutta in this sentence?

Abstract nouns ending in -uus or -yys have a special pattern. When you put them in the partitive case, the ending becomes -utta or -yttä. Therefore, nuoruus becomes nuoruutta.

Why does youth need to be in the partitive case (nuoruutta) here?

There are actually two rules working together here! First, the verb arvostaa (to appreciate) is a partitive verb, meaning its object always takes the partitive case. Second, this is a negative sentence (ei arvostanut), and the object of a negative verb is always in the partitive anyway.

Why does the verb arvostanut end in -nut instead of using a past tense -i- marker?

Because it is a negative sentence in the past tense. To say someone did not do something, you use the negative verb (here, ei) followed by the past participle of the main verb, which ends in -nut or -nyt for singular subjects. You only see the -i- marker in positive past tense sentences (like Hän arvosti for He appreciated).