Learn Finnish: He eivät lisänneet humalaa tähän kattilaan mäskiä. - They did not add hops to this pot of mash.

This is the core of the current lesson! In Finnish, when you have a container (like kattila) holding a substance (like mäski), the substance is put into the partitive case (mäskiä) and acts as a modifier. Even when the container changes case—here it is in the illative (kattilaan) because of the verb—the substance stays locked in the uninflected partitive.

Why is kattila in the illative case (kattilaan)?

The verb lisätä (to add) requires its target destination to be in the illative case (the 'into' form). It answers 'into what' the hops were added.

Is humalaa in the partitive case because the sentence is negative?

Yes, but it would actually be in the partitive even if the sentence were positive! In negative sentences, the direct object is always partitive. However, humala (hops) is used here as an uncountable mass noun, so a positive sentence would also use the partitive: He lisäsivät humalaa (They added some hops).

I recognize the verb lisätä, but why is it written as lisänneet here?