Learn Finnish: Tutkijat ovat löytäneet uusia planeettoja. - The researchers have found new planets.

In Finnish, the object of a sentence is in the partitive case if it represents an unknown or indefinite quantity (like "some new planets"). Even though you are learning the genitive plural right now, the genitive case is only used for a plural object when it refers to a specific, total amount (like "all of the new planets"). Since they found an indefinite number of planets here, we must use the partitive plural.

How does the perfect tense ovat löytäneet work here?

To say "have found" in the perfect tense, you need the helper verb olla (to be) and the past participle of the main verb. Because the subject tutkijat (the researchers) is plural, olla becomes ovat. The main verb löytää (to find) then takes the plural past participle ending -neet, giving us ovat löytäneet.

Why does uusia look so different from its base form uusi?

The word uusi (new) belongs to a special group of old Finnish words ending in -si. When forming the partitive plural, the -si changes to -s- before adding the plural marker -i- and the partitive ending -a, resulting in uusia. For comparison, its genitive plural form would be uusien.

Since this lesson is about the genitive plural, how would we write tutkijat and in that case?