Learn Hardcore Finnish: Mökki on pienellä saarella järven keskellä. - The cottage is on a small island in the middle of the lake.

So on here means is, not the English preposition on. The idea of on (top of) is expressed by the case ending -llä in pienellä saarella, not by the word on.

Why is there no word for the or a before mökki?

Finnish has no articles (no words like a/an or the). Nouns stand alone, and definiteness or indefiniteness is understood from context.

mökki can mean a cottage or the cottage.
The exact English article you choose in translation depends on what makes sense in the context, not on anything in the Finnish grammar here.

What case is pienellä saarella, and why is it used?

Both pienellä and saarella are in the adessive case (ending -lla / -llä).

pieni → pienellä
saari → saarella

The adessive often corresponds to English on or at, especially for surfaces or locations thought of as points/areas:

pöytä (table) → pöydällä = on the table
asemalla = at the station
saarella = on (the) island

So pienellä saarella means on a small island. Finnish uses a case ending instead of a separate word like on.

Why do both pienellä and saarella have the same ending -llä?

In Finnish, adjectives must agree with the noun they modify in:

number (singular/plural)
case (nominative, adessive, etc.)

Here:

noun: saari → saarella (adessive singular)
adjective: pieni → pienellä (adessive singular to match)

So you cannot say pieni saarella; you have to say pienellä saarella, with both in the same case.

What does järven mean, and why is it not just järvi?

järvi means lake in the basic (nominative) form.
järven is the genitive form, roughly of the lake.

järvi (nominative) = lake
järven (genitive) = of the lake

The postposition keskellä (in the middle of) typically takes a genitive in front of it:

talon keskellä = in the middle of the house
metsän keskellä = in the middle of the forest
järven keskellä = in the middle of the lake

So järven is required by keskellä.

What exactly is keskellä? A noun, an adjective, or something else?

keskellä functions as a postposition here, meaning in the middle (of).

Historically it’s related to the noun kesk(i) (middle/center) plus the adessive ending -llä, but in modern usage it behaves like a separate word that:

comes after the genitive noun,
and means in the middle of that noun:

Examples:

aukean keskellä = in the middle of the clearing
parkin keskellä = in the middle of the parking lot
järven keskellä = in the middle of the lake

So syntactically: järven (genitive) + keskellä (postposition).

Why does Finnish say järven keskellä instead of using a preposition like in the middle of the lake?

Finnish typically uses:

case endings (like -llä, -ssa, -sta)
postpositions (like keskellä, päällä, alla)

instead of separate prepositions placed before the noun (like in, on, under in English).

järven keskellä corresponds to English in the middle of the lake, but structurally it is:

järven = lake + genitive = of the lake
keskellä = in the middle

So Finnish builds the same idea with a genitive + postposition, not with a preposition phrase.

Could you also say järvessä to mean in the lake? How is that different from järven keskellä?

Yes, järvessä (inessive case) simply means in the lake (inside lake water / within the lake as a whole):

järvi → järvessä = in the lake

järven keskellä is more specific:

järvessä = somewhere in the lake
järven keskellä = in the middle of the lake (specifically the central area)

So järvessä is a general location; järven keskellä emphasizes the central position.

Why is it saarella (with -lla) and not saaressa (with -ssa)?

Finnish has two main sets of local cases:

internal (inside): -ssa (inessive), -sta, -Vn
external (on/at): -lla (adessive), -lta, -lle

For islands and certain kinds of places, Finnish uses external cases, thinking of them as surfaces or points:

saarella = on the island / at the island
torilla = at/on the market square
asemalla = at the station

saaressa (internal -ssa) would feel more like inside the island, which is unusual unless you mean literally inside its interior (e.g. some geological or metaphorical context). So for normal location, saarella is the natural form.

Can the word order change, for example to Mökki on järven keskellä pienellä saarella?

Yes, Finnish word order is relatively flexible. Both:

Mökki on pienellä saarella järven keskellä.
Mökki on järven keskellä pienellä saarella.

are grammatical and mean essentially the same thing.

Subtle differences:

The part that comes earlier often carries slightly more emphasis or is treated as more “given” or central information.
In the original, the focus is first on on a small island, then in the middle of the lake specifies where that island is.
In the reordered version, you first highlight in the middle of the lake, and then clarify that this middle position is on a small island.

But in everyday speech, the difference is tiny here; both are normal.

Is mökki just any house, or does it have a special meaning?

mökki is usually a small cottage, often a summer cottage or holiday cabin, frequently by a lake or in the countryside.

talo = house (neutral, any residential building)
mökki = cottage / cabin (smaller, simpler, often rustic, often for vacations)

In this sentence, mökki naturally suggests a holiday cottage on a lake island, a very typical Finnish image.

How do you pronounce the special vowels ö and ä in this sentence?

In the sentence Mökki on pienellä saarella järven keskellä:

ö (in Mökki) is like the vowel in French peu, German schön, or somewhat like the vowel in English bird (but with rounded lips).
ä (in pienellä, saarella, järven, keskellä) is like a in English cat, but held more pure and forward in the mouth.

So roughly:

Mökki ≈ MEU-kki (with rounded lips for ö)
pienellä ≈ pee-eh-ne-lla
saarella ≈ saa-rella (long aa)
järven ≈ YAIR-ven (with ä as in cat)
keskellä ≈ kes-kel-la (both e like in English pet, ä like in cat)

Why are there no English-like prepositions such as on, in, or of in this sentence?

Finnish largely encodes those meanings in:

case endings:
- -lla / -llä = on/at (pienellä saarella)
- genitive -n = of (järven)
postpositions:
- keskellä = in the middle (of)

So instead of:

on a small island in the middle of the lake

Finnish has:

mökki on pienellä saarella (the cottage is on a small island)
järven keskellä (in the middle of the lake)

The meanings on, in, of are all expressed by morphology and word combinations, not separate prepositions.