Learn Finnish: Työntekijä putosi tähän tynnyriin mallasta. - The employee fell into this barrel of malt.

This is the classic "container and contents" structure. The container (tynnyri) changes case to fit the sentence's grammar—here it is illative (tynnyriin) because someone fell into it. However, the contents (mallas, meaning malt) stay firmly in the partitive case (mallasta) to show what the container holds. The modifier never changes case to match the head noun.

What is the base form of mallasta, and how is the partitive formed for this word type?

The nominative (base form) is mallas (malt). It belongs to the group of words ending in -as or -äs. For this word type, the partitive is very straightforward: you just add -ta or -tä directly to the nominative form, giving us mallasta.

Why do we use the illative case (tähän tynnyriin) instead of the inessive (tässä tynnyrissä)?

The verb pudota (to fall) expresses a movement or a change of location ending up somewhere. In Finnish, verbs of movement require a "to where" case (illative or allative) rather than a "where" case (inessive or adessive). Because the employee fell into the inside of the barrel, we use the illative.

Could we put mallasta before tynnyriin, similar to how we might say "malt barrel" in English?