Learn Finnish: Tämä hiiri on ovela. - This mouse is cunning.

You would use the conditional form of the verb olla, which is olisi. The phrase would be Vaikka tämä hiiri olisi kuinka ovela... Notice how kuinka often shifts to sit right in front of the adjective it modifies.

Why use ovela here instead of other words for 'smart' like fiksu or viisas?

Ovela specifically means cunning, sly, or crafty. It implies someone uses tricks or clever strategies, which perfectly fits a mouse avoiding a trap. Words like fiksu (smart) or viisas (wise) imply general intelligence or life wisdom, which would sound out of place for a mouse.

I sometimes still mix up the stems of nouns ending in -i. How does hiiri behave in the partitive and plural?

Hiiri belongs to the group of older Finnish words. In the partitive singular, the final vowel drops out entirely, making it hiirtä. When forming plurals, the stem ends in -e-, so you get forms like hiiret (nominative plural) and hiiriä (partitive plural, where the plural -i- merges with the e-stem).

If I wanted to describe the mouse's actions and say it behaves 'cunningly', how would I change ovela?