Learn Finnish: Kateus on kauheaa.

Because the subject kateus (envy) is an abstract concept. When the subject of a sentence is a mass noun or an abstract concept, the adjective describing it (the subject complement) must be in the partitive case.

If the sentence is about an abstract concept, why is the subject kateus in the nominative case?

In basic descriptive sentences where A is B, the subject itself is almost always in the nominative case. The abstract nature of the subject only forces the describing adjective into the partitive case, not the subject itself.

Would it be wrong to say Kateus on kauhea?

Yes, this would sound incorrect. Using the nominative kauhea implies that the subject is a specific, countable thing (like a book or a dog). Because envy is a general, uncountable feeling, you must use the partitive to describe it.

What kind of word is kateus and how does it relate to other vocabulary?

kateus is an abstract noun meaning envy. In Finnish, you can often create abstract nouns from adjectives by adding the suffix -us or -ys. It is closely related to the adjective kateellinen, which means envious.