Learn Finnish: Jos erehdyt tästä kopiosta, kokoelma koostuu vain väärennöksistä. - If you are mistaken about this copy, the collection consists only of forgeries.

The verb erehtyä (to be mistaken, to err) requires the elative case (-sta/-stä) when you are expressing what someone is mistaken about. Even though it's not a physical separation, this mental "straying from" the truth triggers the same elative rection you are practicing in this lesson.

Does koostua also require the elative case?

Yes! Just like verbs of separating and depending, verbs describing what something is made of or consists of take the elative case. Here, the collection "consists of forgeries", so the verb koostuu forces the word for forgeries into the elative: väärennöksistä.

How do we get from the basic dictionary form of "forgery" to väärennöksistä?

The dictionary form is väärennös. Nouns ending in -os/-ös change to -kse- in their stem, giving us väärennökse-. To make it plural, the -e- is dropped before the plural -i-, creating the plural stem väärennöksi-. Finally, we add the elative ending -stä to get väärennöksistä.

Is väärennös related to the word väärä (wrong)?