Learn Hardcore Finnish: Pöydän ja ikkunan välillä on tuoli. - Between the table and the window there is a chair.

Literally, the structure is something like:
“on the space of the table and of the window is a chair” → “Between the table and the window there is a chair.”

So whenever you use välillä (or similar postpositions like lähellä “near”, takana “behind”), you put the related nouns in the genitive:

talon takana – behind the house
pöydän ja ikkunan välillä – between the table and the window

What exactly is välillä, and why does it come after the nouns instead of before like English “between”?

Välillä is a postposition, derived from the noun väli (“gap, space, interval”) in the adessive case (-lla/-llä). Literally: “on/at the space”.

väli = (a) space, gap
välillä = “in the (on-the) space (between)”

Finnish has both prepositions (before the noun) and postpositions (after the noun), but postpositions are more common in basic location expressions. So instead of:

“between the table and the window”

Finnish does:

pöydän ja ikkunan välillä
literally “the table’s and the window’s between”

Other common postpositions:

talon edessä – in front of the house
talon vieressä – next to the house
talon takana – behind the house

What is the difference between välillä and välissä? Could I say pöydän ja ikkunan välissä?

Both välillä and välissä can mean “between” and both are used with genitive nouns:

pöydän ja ikkunan välillä
pöydän ja ikkunan välissä

They are both grammatically correct here. There is a subtle nuance:

välissä (inessive -ssa/-ssä) literally means “in the gap/inside the space” and is especially common for concrete physical space.
välillä (adessive -lla/-llä) can be used for both concrete and more abstract “between” (between people, times, choices, etc.) and often emphasizes the span or range.

In everyday speech, pöydän ja ikkunan välissä on tuoli is very natural for physical location, but välillä in the original sentence is also fine and understandable. As a learner, you can treat them as near-synonyms for basic spatial “between” and learn the more delicate differences gradually.

Can I say “Tuoli on pöydän ja ikkunan välissä” instead? How is that different from “Pöydän ja ikkunan välillä on tuoli”?

Yes, you can. The difference is mostly one of focus and sentence type:

Pöydän ja ikkunan välillä on tuoli.
This is an existential sentence (“there is/are” type).
- Focus: introducing the existence or presence of a chair in that location.
- Typical answer to “Is there a chair somewhere here?” or “Where is there a chair?”
Tuoli on pöydän ja ikkunan välissä.
This is a normal “X is Y” sentence.
- Focus: the location of a particular chair already known in the conversation.
- Typical answer to “Where is the chair?”

In English the nuance is similar to:

There is a chair between the table and the window.
vs.
The chair is between the table and the window.

Why is tuoli in the basic form (nominative), not in some special case?

In this existential sentence, tuoli is the subject that is being introduced as existing in a certain place, so it appears in the nominative singular:

tuoli – a/the chair

In positive existential sentences with a countable singular thing and no extra modifiers, the subject is usually nominative:

Pöydän alla on laatikko. – There is a box under the table.
Kadulla on auto. – There is a car on the street.

If you talk about an indefinite amount or many items, you normally use the partitive plural:

Pöydän ja ikkunan välillä on tuoleja. – There are (some) chairs between the table and the window.

Or with a number:

Pöydän ja ikkunan välillä on kaksi tuolia. – There are two chairs between the table and the window.

What kind of sentence pattern is “Pöydän ja ikkunan välillä on tuoli” in Finnish, and why does the subject come last?

It’s a typical existential sentence (Finnish: eksistentiaalilause), whose basic pattern is:

[Location] + on/oli/… + [new thing]

So here:

Pöydän ja ikkunan välillä – the location phrase
on – the verb (olla “to be”)
tuoli – the newly mentioned thing, the subject

Finnish uses this structure to express English “there is / there are” without a word like “there”:

Pihalla on koira. – There is a dog in the yard.
Kadulla on paljon autoja. – There are many cars on the street.

The location comes first, because it sets the scene. The new or important item (here tuoli) often comes last for information focus.

What does on mean here? Is it the same word as English “on”?

No, it is not related to English preposition “on”.

on here is the 3rd person singular form of the verb olla – “to be”.
- olla – to be
- minä olen – I am
- sinä olet – you are
- hän on – he/she is
- me olemme – we are
- te olette – you (pl.) are
- he ovat – they are

In existential sentences like this, on can translate as both “is” and “are” depending on context:

Pöydän ja ikkunan välillä on tuoli. – There is a chair…
Pöydän ja ikkunan välillä on kaksi tuolia. – There are two chairs…

English “on” as a preposition is usually translated in Finnish with -lla/-llä, päällä, or something else, depending on context (e.g. pöydällä = “on the table”).

How do you pronounce the ö and ä in pöydän?

Approximate pronunciation of pöydän: [PÖY-dan].

Vowels:

ö
- A front vowel (lips rounded), similar to French eu in peur or German ö in schön.
- Not like English “o”; your tongue is forward, lips rounded.
ä
- A front vowel, like a slightly more open version of the a in English “cat”, or German ä in Mädchen.
- Not like English “ah” in “father” (that would be closer to Finnish a).

And the diphthong öy is a smooth glide from ö to y (y like French u in lune, a high front rounded vowel).

Stress is always on the first syllable in standard Finnish:

PÖY-dän

What is going on with pöytä → pöydän and ikkuna → ikkunan? Why does pöytä suddenly have a d?

This is due to consonant gradation, a regular sound change in Finnish that affects many words when you add case endings.

For pöytä:

Basic form (nominative): pöytä
Genitive singular: pöydän

Here, t changes to d between vowels:

t → d in some forms (weak grade)
pöytä (strong grade) → pöydän (weak grade)

You will see similar patterns elsewhere:

matto (a carpet) → maton (of the carpet) – tt → t
puku (a suit) → puvun (of the suit) – k → v

For ikkuna:

ikkuna → ikkunan

There is no gradation change here; the stem ikkuna- stays the same, and you simply add -n for the genitive.

As a learner, you mostly need to memorize which words undergo which pattern; dictionaries and grammar tables usually show the key forms.

How would I say “Between the tables and the windows there is a chair” (with both “table” and “window” in the plural)?

You need the genitive plural of both nouns:

pöytä → genitive plural pöytien
ikkuna → genitive plural ikkunoiden (also ikkunain/ikkunoitten in some styles, but ikkunoiden is very common)

Then attach välillä or välissä:

Pöytien ja ikkunoiden välissä on tuoli.
Between the tables and the windows there is a chair.

You could also use välillä:

Pöytien ja ikkunoiden välillä on tuoli.

Both are acceptable; välissä is very natural for a concrete physical in-between space.

How do I say “There are two chairs between the table and the window” in Finnish?

You keep the location phrase the same, then add kaksi (“two”) and put tuoli in the partitive plural (tuolia), because numbers 2 and above require partitive plural:

Pöydän ja ikkunan välissä on kaksi tuolia.
or
Pöydän ja ikkunan välillä on kaksi tuolia.

Structure:

pöydän ja ikkunan välissä/välillä – between the table and the window
on – is/are
kaksi tuolia – two chairs (kaksi = head, tuolia = partitive plural)

Can välillä be used only for physical space, or also for people, time, etc.?

Välillä is quite flexible; it’s used for physical, social, and temporal “between”.

Examples:

Physical:
- Pöydän ja ikkunan välillä on tuoli. – A chair is between the table and the window.
Between people:
- Lasten välillä on riita. – There is a quarrel between the children.
- Meidän välillämme on luottamusta. – There is trust between us.
Between times / events:
- Kello yhden ja kahden välillä. – Between one and two o’clock.
- Maanantain ja perjantain välillä. – Between Monday and Friday.

There is also an idiomatic pattern “välillä X, välillä Y”:

Välillä sataa, välillä paistaa. – Sometimes it rains, sometimes it shines.

So välillä is not limited to physical space; it’s widely used in abstract “between” meanings too.