Learn Finnish: He korjaavat pahoja risteyksiä. - They are fixing bad intersections.

In Finnish, the object of a sentence takes the partitive case when the action is ongoing, continuous, or incomplete. Because they "are fixing" the intersections right now, the process isn't finished yet, so we use the partitive plural object pahoja risteyksiä. If they were completely finishing a specific set of intersections, the object would be in the nominative plural: He korjaavat pahat risteykset (They will fix the bad intersections).

How does the word paha (bad) become pahoja in the partitive plural?

The partitive plural is made by adding the plural marker -i- (which becomes -j- between vowels) and the partitive ending -a or -ä. For two-syllable words ending in -a like paha, that final -a changes to an -o- before the plural marker to make it easier to pronounce. So, paha becomes paho- + j + a = pahoja.

What is the base form of risteyksiä, and why does its spelling change so much?

The base form is risteys (intersection). Nouns ending in -us or -ys change that ending to -ukse- or -ykse- before you add most endings. The basic stem here is risteykse-. When we add the plural -i- and partitive -ä, the final -e- drops out, leaving us with risteyksiä.

Why does pahoja have to be in the partitive plural just like ?