Learn Finnish: He rakentavat kalliita siltoja. - They are building expensive bridges.

There are two reasons! First, the action is ongoing. The verb rakentaa (to build) takes a partitive object when the process is not yet finished. If the action were completed, the object would be in the nominative plural (kalliit sillat). Second, it expresses an indefinite amount (meaning "some expensive bridges" rather than "the expensive bridges").

Why does the word silta change to siltoja? Where does the "o" come from?

When forming the plural stem, a short -a at the end of a two-syllable word usually changes to -o- before the plural -i-. So silta becomes the stem siltoi-. Then we add the partitive ending -a. Because Finnish doesn't like having three vowels in a row, the -i- changes to -j-, giving us siltoja.

How do we get kalliita from the basic word for expensive (kallis)?

For words ending in -is (like kallis or kaunis), the plural stem is formed by replacing the -is with a long vowel, giving us the stem kallii-. To form the partitive plural, we simply add the partitive ending -ta to this stem, resulting in kalliita.

Why can't we just say kallis siltoja?