Learn Finnish: Maalaan seinän uudella siveltimellä. - I paint the wall with a new paintbrush.

While the adessive case (-lla/-llä) is often used to show location (meaning "on"), here it is used to express the instrument or tool used to perform an action. In English, we translate this usage as "with" (with a paintbrush).

Why is the word for "new" written as uudella instead of uusi?

In Finnish, adjectives must agree with the noun they describe. Because the paintbrush is in the adessive case (siveltimellä), the adjective must also take the adessive case. The stem of uusi is uude-, and adding the ending gives us uudella.

The dictionary word for paintbrush is sivellin. Why does it change so much to become siveltimellä?

Nouns ending in -in change to -ime- to form their stem (just like puhelin becomes puhelime-). Additionally, this word undergoes reverse consonant gradation: the weak double consonant ll in the basic form sivellin becomes a strong lt in the stem, resulting in siveltimellä.

Why is the wall (seinä) written as seinän here instead of the partitive seinää?