Learn Finnish: Vuokra on kallis, mutta huone on uusi. - The rent is expensive, but the room is new.

Because this sentence doesn't express an obligation! You use a genitive subject with verbs like täytyy (must). This is just a standard descriptive sentence using on (is), so the subjects (vuokra and huone) stay in their basic dictionary form.

Why is there a comma before mutta in this sentence?

In Finnish, you almost always put a comma before mutta (but) when it connects two complete thoughts that each have their own subject and verb. Since vuokra on kallis and huone on uusi could both stand alone as full sentences, they must be separated by a comma.

Is huone a normal noun, or does its -e ending mean it changes in special ways?

huone is an -e stem word, which is a special and very common group of words in Finnish. While it is in its simple basic form here, if you were to add an ending to it—like saying 'in the room'—that final -e would double to make the stem huonee- before adding the ending (huoneessa).

If the rent and room were plural, would kallis and uusi change too?