Learn Finnish: Rauta on raskasta.

Because the subject rauta (iron) is a mass noun, meaning it is an uncountable material. In Finnish, when the subject is a mass noun or an abstract concept, the adjective describing it (the subject complement) must be in the partitive case.

Since rauta is an uncountable material, why isn't it also in the partitive case here?

Because rauta is the subject of the sentence, and we are making a general statement defining all iron. Subjects are typically in the nominative case for general definitions. You would only use the partitive for the subject if you were talking about an unspecified amount of it, such as in the sentence Täällä on rautaa (There is some iron here).

What is the rule for forming the partitive raskasta from the dictionary form raskas?

For adjectives and nouns ending in -as or -äs, you form the partitive case simply by adding the ending -ta or -tä directly to the dictionary form. There is no stem change required, so raskas easily becomes raskasta.

If I am holding a specific piece of iron, would I still use the partitive raskasta to describe it?