Here's the answer copy-pasted from the FAQ I've put together. Partitive case is applied to...
- ...noun phrases modified by a singular numeral (except “yksi”). “Kaksi poroa” = “Two reindeer”
- ...noun phrases modified by an expression of an amount. “Suomessa on monta järveä” = “There are many lakes in Finland”
- ...plural noun phrases with indefinite referents in existential clauses and ownership clauses even when not modified by a numeral or an expression of an amount, i.e. the number/amount is unspecified. In other words, it is used in words that would usually be modified by “some” or “any” in an English translation. If the referents are definite, nominative case is used instead. “Kaupassa on karjalanpiirakoita” = “There are (some) Karelian pasties in the shop”
- ...mass nouns (usually). “Pullossa on viinaa” = “There is liquor in the bottle”
- ...the target of an ongoing action. “Rakennamme saunaa” = “We are building a sauna”
- ...the target of an irresultative action. “Halasin karhua” = “I hugged a bear”
- ...negated noun phrases in an existential clause or an ownership clause. “Minulla ei ole autoa” = “I don’t have a car”
- ...the subject of an existential clause or an ownership clause when the clause is in the form of a question and the inquirer either isn’t sure what answer to expect or expects a negative answer. When the inquirer is sure that the answer will be positive, nominative case is used instead. “Onko teillä juustohöylää?” = “Do you have a cheese slicer?”
- ...noun phrases preceded by (most) prepositions. “Ilman pipoa” = “Without a woolly hat”
- ...probably some other not-so-common situations that I can’t think of right now.
Also, "pehmeä pullaa" wouldn't make sense because the partitive case there is only applied to the noun. It should be applied to the entire noun phrase, which would include the adjective as well.