So for example: Ostan jonkin verran leipää / I'm buying some bread.
Here the base form Leipä transforms to Leipää because it is part of an action. I at least think that's how it goes.
Or because it's an undefined part of a bigger whole. Like "lautasella on puuroa", there's porridge on the plate -> there's an undifined part of a bigger unity of porridge on the plate, compared to "puuro on lautasella", the porridge is on the plate -> it's all the porridge that's been cooked or an otherwise specified amount/sort of porridge.
Finnish: tai koska se on epämääräinen osa isompaa kokonaisuutta. Niin kuin "lautasella on puuroa", there's porridge on the plate -> lauatsella on määrittämätön osa isommasta puurokoknaisuudesta, verrattuna lauseeseen "puuro on lautasella", The porridge is on the plate -> kyseessä on koko keitetty puuroannos tai muuten määritelty puuromäärä/-laji.
It has inspired me with a new plan for studying Finnish. How to say,
"Honestly, I tried learning some Finnish, but I hurt myself. Do you speak English?"
"Oikeasti, yritin oppia vähän suomea, mutta satutin itseni. Puhutko englantia?"