Learn Finnish: En voi allekirjoittaa sopimusta, sillä tämä vaatimus on mahdoton. - I cannot sign the agreement, because this demand is impossible.

It is formed by taking the verb vaatia (to demand) and adding the -mus suffix. This suffix turns verbs into nouns that represent the result of the action or an abstract concept, so "to demand" becomes "a demand".

Is sopimusta also an example of the -mus suffix?

Yes! Its base form is the noun sopimus (agreement), which is derived from the verb sopia (to agree) plus the -mus suffix. It is in the partitive case here (sopimusta) because it is the object of a negative verb phrase (En voi allekirjoittaa).

If the base word is sopimus, why is the partitive form sopimusta instead of using a stem like sopimukse-?

For nouns ending in -mus (and other -us/-ys words where a consonant comes before the ending), the partitive case is an exception: you simply add -ta/-tä directly to the basic dictionary form. The -ukse- stem is used for most other cases, such as the genitive (sopimuksen) or the illative (sopimukseen).

Could I use koska instead of sillä for "because"?

Yes, they are practically interchangeable in meaning here. Koska is very common in both spoken and written Finnish. Sillä is slightly more formal and often acts as a connecting word to add a justification or explanation to the previous clause.