Olen kaupassa ostamassa omenoita, shampoota ja uuden harjan.

Finnish has three “inner” local cases for “in/into/out of”:

• kauppaan = into the store
• kaupassa = in the store
• kaupasta = out of/from the store

Adessive -lla/-llä (e.g., kaupalla) means “at/on,” and while it’s used with some places (especially named ones, e.g., Prismalla) or idioms, for a generic “store” the natural choice here is kaupassa (“in the store”).

Each item takes the object form that fits its meaning:

• omenoita: partitive plural = an indefinite quantity of apples.
• shampoota: partitive singular = an indefinite amount of a mass noun (shampoo).
• uuden harjan: genitive singular (the “total object”) = one specific, delimited brush.

It’s fine to mix these in one list; each noun chooses the case that matches how it’s being bought.

Yes:

• uutta harjaa (partitive) = non-delimited/ongoing/unspecified; you’re in the process of getting a new brush, not focusing on a particular completed result.
• uuden harjan (genitive total object) = a specific, single brush as a completed target.

Both can occur with olla … -massa; the choice signals your perspective on the result.

After verbs like ostaa, objects appear in object case, not nominative. Here it’s a singular “total” object:

• Adjective: uusi → uuden (note the -si → -de stem change)
• Noun: harja → harjan The adjective agrees with the noun in case and number: uuden harjan.

• omenoita (partitive plural) = some apples (indefinite amount).
• omenat (plural total object) = the apples (a specific, delimited set).

With the progressive-like olla … -massa, partitive is the neutral choice for “some apples.” Using omenat here sounds odd unless a very specific, known set is meant—and even then, Finnish tends to prefer a different structure (e.g., Ostan ne omenat or Menen ostamaan omenat).

Partitive plural patterns vary. For omena, the partitive plural is omenoita (with an inserted -o- plus -ita). It’s a common pattern for this noun type. Compare:

• omena → omenoita
• koira → koiria (a different pattern) Partitive plurals are irregular enough that memorizing common nouns helps.

Loanword declension:

• Nominative: shampoo
• Genitive: shampoon
• Partitive singular: shampoota
• Nominative plural: shampoot

Here we need partitive singular (shampoota) for an indefinite amount. You may hear people pronounce it with [ʃ] or as [s]; both occur. There’s also an older variant sampoo in some registers.

Use a motion verb + the -maan/-mään form:

• Menen kauppaan ostamaan omenoita, shampoota ja uuden harjan. That expresses purpose (“to buy”), not an ongoing activity at the location.