Learn Hardcore Finnish: Olen kaupassa ostamassa omenoita, shampoota ja uuden harjan. - I am at the store buying apples, shampoo, and a new brush.

Olla + the -massa/-mässä form (the inessive of the MA-infinitive) highlights being in the middle of an activity, often tied to a location. It’s like English “I’m in the store buying…”. Plain present ostan is neutral about “right now/in progress” and can also mean a near-future intention (“I’ll buy”), without foregrounding the ongoing activity or location.

Why is it ostamassa and not ostamaan?

-massa/-mässä (inessive) goes with stative/location-type verbs like olla to mean “in the process of doing”: Olen kaupassa ostamassa.
-maan/-mään (illative) goes with motion verbs to express purpose: Menen kauppaan ostamaan (“I’m going to the store to buy…”).

Why is it kaupassa and not kaupalla?

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”).

Why are the objects in different cases: omenoita, shampoota, but uuden harjan?

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.

Could I use the partitive with harja too (uutta harjaa)? What’s the nuance?

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.

Why is it uuden harjan and not uusi harja?

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.

Why omenoita and not omenat?

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).

How is omenoita formed from omena?

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.

What’s going on with shampoota? Why that spelling and ending?

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.

Is it okay to mix partitive and genitive objects in the same list?

Yes. Each coordinated item carries the case that matches its own semantics. So omenoita, shampoota ja uuden harjan is perfectly natural.

Can I say Olen ostamassa kaupassa instead of Olen kaupassa ostamassa?

It’s grammatical, but the default, smooth word order places the location before the -massa phrase: Olen kaupassa ostamassa. Saying Olen ostamassa kaupassa can sound like you’re specifically contrasting the place (“it’s in the store that I’m doing the buying”).

Do I need the pronoun minä?

No. Finnish normally drops subject pronouns because the verb form shows the person. Olen already means “I am.” Minä olen is used for emphasis or contrast.

Should there be a comma before ja in the list?

No. Finnish does not use the Oxford comma. The natural list is omenoita, shampoota ja uuden harjan (no comma before ja).

How would I say I’m going to the store to buy these things?

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.