Learn Finnish: Kupari on kallista. - Copper is expensive.

Because the subject, kupari (copper), is a mass noun, meaning it is an uncountable substance. In Finnish, when you use the verb olla (to be) to describe a mass noun or an abstract concept, the describing adjective (the subject complement) must be in the partitive case.

Could I ever say 'Kupari on kallis'?

Usually, no. You would only use the nominative kallis if you were describing a countable, individual object. For example, Sormus on kallis (The ring is expensive). Since copper as a material cannot be counted, its describing adjective requires the partitive.

Wait, shouldn't the subject kupari also be in the partitive case because it is a mass noun?

No, as the subject of this specific type of sentence, kupari stays in the basic nominative case. The partitive rule here only applies to the adjective describing it. You use a partitive subject for mass nouns mostly in existential sentences expressing an undefined amount, like Pöydällä on kuparia (There is some copper on the table), but here we are stating a general fact about the material as a whole.

How do you form the partitive for words like kallis?