Aseman vieressä on pieni kioski, josta ostan vettä.

Because vieressä is a postposition, and many Finnish postpositions require the word before them to be in the genitive case.

• asema = station
• aseman = of the station / station’s

So:

• aseman vieressä = next to the station

This is a very common pattern in Finnish:

• talon edessä = in front of the house
• kaupan takana = behind the shop
• koulun lähellä = near the school

Vieressä means beside / next to.

It comes from vieri (side) and behaves as a postposition-like location word. In Finnish, location expressions are often built differently from English:

• English: next to the station
• Finnish: station-GEN beside

So aseman vieressä literally works like at the side of the station.

You will also often see related forms:

• vieressä = next to, beside
• viereen = to beside, to the side of
• vierestä = from beside, from next to

Example:

• Menen aseman viereen. = I go next to the station.
• Tulen aseman vierestä. = I come from beside the station.

Both are possible, but they feel a little different.

Aseman vieressä on pieni kioski is a very natural Finnish way to introduce the existence of something in a place. It is similar to saying:

• Next to the station, there is a small kiosk.

This word order is common when the sentence is about what exists somewhere.

By contrast:

• Pieni kioski on aseman vieressä.

This puts more focus on the kiosk itself, as if it is already known and you are telling where it is.

So:

• Aseman vieressä on pieni kioski = natural there is structure
• Pieni kioski on aseman vieressä = the small kiosk is next to the station

Josta is the elative form of the relative pronoun joka.

• joka = who / which / that
• josta = from which / from where

Here it refers back to kioski:

• pieni kioski, josta ostan vettä
• literally: a small kiosk, from which I buy water

In natural English, this becomes:

• a small kiosk where I buy water
• or a small kiosk from which I buy water

Finnish often uses a case ending on the relative pronoun instead of a separate preposition like English does.

Because Finnish does not use where in the same way English does in relative clauses.

English says:

• the kiosk where I buy water

Finnish expresses this with the relative pronoun joka plus a case ending:

• kioski, josta ostan vettä

The case ending shows the relationship:

• jossa = in which
• johon = into which
• josta = from which

With places of buying, Finnish commonly uses ostaa jostakin = buy from somewhere. That is why josta is used.

Because vettä is in the partitive case, which is very common after verbs like ostaa when you mean an unspecified amount or a substance in general.

• vesi = water
• vettä = water (partitive)

So:

• ostan vettä = I buy some water / I buy water

If you said:

• ostan veden

that would usually mean I buy the water or I buy a specific quantity/unit of water, which sounds much more definite and less natural in this context.

Finnish uses the partitive a lot with:

• substances: juon kahvia = I drink coffee
• ongoing/incomplete actions in some contexts
• unspecified quantities: ostan leipää = I buy bread / some bread

Yes. In Finnish, ostaa often works with the source of the purchase:

• ostaa kaupasta = buy from the shop
• ostaa kioskista = buy from the kiosk

That is why the relative clause uses josta:

• kioski, josta ostan vettä
• literally: kiosk from which I buy water

Even though English often just says at or where, Finnish naturally uses from here.

Finnish has no articles like English a/an/the.

So pieni kioski can mean:

• a small kiosk
• the small kiosk

The exact meaning depends on context.

In this sentence, because the kiosk is being introduced for the first time, English naturally translates it as a small kiosk.

Finnish shows definiteness mostly through:

• context
• word order
• pronouns if needed

For example:

• se kioski = that/the kiosk

But here plain pieni kioski is enough.

Because pieni kioski is the subject of the clause, and here it is in the nominative singular.

• pieni = small
• kioski = kiosk

So:

• pieni kioski = a small kiosk

In Finnish, adjectives usually agree with the noun in case and number. Since kioski is nominative singular here, pieni is too.

Compare:

• pieni kioski = a small kiosk
• pienen kioskin = of a small kiosk / a small kiosk in genitive-accusative-like form
• pienessä kioskissa = in a small kiosk

The sentence has two parts:

1. Aseman vieressä on pieni kioski

   • Next to the station there is a small kiosk

2. josta ostan vettä

   • from which I buy water
   • more natural English: where I buy water

So the full structure is:

• [place] + on + [thing], [relative clause]

This is a very common Finnish pattern:

• Kadun kulmassa on kahvila, jossa tapaan ystäviäni.
  • On the corner of the street there is a café where I meet my friends.

Because josta ostan vettä is a relative clause describing pieni kioski.

Finnish normally separates relative clauses with a comma:

• pieni kioski, josta ostan vettä

This is similar to how English may punctuate added descriptive information, although Finnish comma rules are often a bit stricter in subordinate clauses than English ones.

So the comma helps show that the second part is explaining which kiosk is meant.

Yes, and learners often wonder about this.

There is a subtle difference:

• josta ostan vettä = from which I buy water
• jossa ostan vettä = in which I buy water

With kioski, Finnish usually prefers ostaa jostakin when talking about the place you buy something from, so josta is very natural.

jossa would sound more like emphasizing being inside the kiosk while buying, which is less idiomatic in this context.

So the best choice here is:

• josta ostan vettä

Here are the main dictionary forms:

• aseman → asema = station
• vieressä → related to vieri / expression vieressä = next to, beside
• on → olla = to be
• pieni = small
• kioski = kiosk, stand
• josta → joka = which / that
• ostan → ostaa = to buy
• vettä → vesi = water

This is useful because Finnish words often appear in sentences in forms that look quite different from the dictionary form.