In Finnish, when you talk about a container or a measure of something (like "a liter of water" or "a barrel of mash"), the word for the container or measure is the main noun, and the substance inside is put into the partitive case.
Inflecting the Container, Not the Contents
The most important rule for these phrases is that the substance always stays in the partitive case, even when the container changes case to show location, direction, or its role in the sentence.
For example, if you say "into a liter of water," the word for "liter" takes the illative ending (litraan), but "water" remains in the partitive (vettä).
This holds true for any grammatical case applied to the container or measure. Whether you are talking about something happening inside a drop (inessive), falling into a barrel (illative), or drinking out of a bottle (elative), the word for the contents remains uninflected.
Modifiers Agree with the Container
If the container has its own adjectives or pronouns (like "this" or "large"), those words do change to agree with the container's case. The substance at the end of the phrase, however, still remains securely in the partitive.