Questions & Answers about Tämä vanha miekka on rautaa.
Why is rautaa in the partitive case here instead of the nominative rauta?
Rauta (iron) is a mass noun, meaning it is a word for a material or substance. In Finnish, when you use a mass noun after the verb olla (to be) to describe what an object is made of, it must take the partitive case. That is why it becomes rautaa.
Would it mean something different if I said Tämä vanha miekka on rauta?
Yes, and it would sound very unnatural! Using the nominative case after the verb olla usually implies a direct, one-to-one identity. Saying it with rauta makes it sound like this specific sword is the abstract concept of iron itself, rather than just a physical object made out of that material.
Why don't the subject words (Tämä vanha miekka) also take the partitive case?
The partitive rule in this lesson only applies to the mass noun describing the material at the end of the sentence. The subject, Tämä vanha miekka, is a specific, countable object (a sword), so it simply stays in the basic nominative case.
If I wanted to say 'This old sword is heavy iron,' how would the word heavy fit in?