Learn Hardcore Finnish: Pöytä on siirretty ikkunan alle. - The table has been moved under the window.

Depending on context, it can also describe a resulting state, a bit like “the table is (in a moved state) under the window”, but grammatically it is the perfect passive, not a simple present.

Why is the passive used here (on siirretty) instead of an active form? Who moved the table?

Finnish uses the impersonal passive when:

the doer is unknown, irrelevant, or obvious from context
you want to focus on the result or object, not on who did it

Pöytä on siirretty ikkunan alle.
→ The table has been moved under the window (by someone).

If you do want to mention who moved it:

Joku on siirtänyt pöydän ikkunan alle.
Someone has moved the table under the window.
He ovat siirtäneet pöydän ikkunan alle.
They have moved the table under the window.

So on siirretty keeps the agent anonymous, just like English “has been moved” without saying by whom.

What is the difference between Pöytä on siirretty ikkunan alle and Pöytä on siirtynyt ikkunan alle?

The difference is who/what causes the movement:

siirtää = to move something (transitive, needs an object)
- Pöytä on siirretty ikkunan alle.
  The table has been moved under the window (by someone).
siirtyä = to move / shift oneself, to be moved (intransitive)
- Pöytä on siirtynyt ikkunan alle.
  Literally The table has moved under the window (itself / on its own / somehow).

In practice:

Use on siirretty when a person or an outside force has put it there.
Use on siirtynyt when the table’s position has changed without emphasizing an agent (e.g. it slid, it shifted during moving, etc.).

Why is it ikkunan alle and not ikkuna alle or ikkunan alla?

There are two key points: genitive case and direction vs location.

Genitive + postposition

In Finnish, many place words work as postpositions and require the noun in the genitive:

ikkunan alle = under the window (to that place)
- ikkunan = of the window (genitive of ikkuna)
- alle = to under

So ikkuna alle is ungrammatical; the correct form is ikkunan alle.

Movement vs static location

alle (direction: to under)
alla (static: under, underneath)

Compare:

Pöytä on siirretty ikkunan alle.
→ The table has been moved to a position under the window.
Pöytä on ikkunan alla.
→ The table is under the window (no motion implied).

So:

ikkunan alle = movement to a position under the window
ikkunan alla = static position under the window

Is ikkunan alle a preposition or a postposition phrase? How does it work?

Ikkunan alle is a postposition phrase.

Structure:

ikkunan (genitive) + alle (postposition, directional)

In Finnish:

A preposition comes before the noun:
- ennen taloa = before the house
A postposition comes after the noun:
- talon alle = under the house (to under the house)
- ikkunan alle = under the window (to under the window)

Most basic spatial words like alla / alle, päällä / päälle, edessä / eteen, takana / taakse are used as postpositions with the noun in the genitive:

pöydän alla = under the table
talon taakse = (to) behind the house
auton eteen = (to) in front of the car

Why is pöytä in the nominative and not pöydän or pöytää in Pöytä on siirretty ikkunan alle?

Here pöytä behaves like a subject in the perfect passive construction.

In sentences of the type:

Pöytä on siirretty.
Ovi on avattu.
Auto on korjattu.

the affected thing is in the nominative singular:

pöytä
ovi
auto

You could think of these like English:

The table has been moved.
The door has been opened.
The car has been repaired.

So:

pöytä (nominative) fits because the event is completed and the whole table has been moved.
A partitive form would change the meaning:
- Pöytää on siirretty.
  = The table has been (being) moved (around) / There has been some moving of the table.
  Focus on ongoing/partial activity, not on a completed result.

Pöydän would be an object form in an active sentence, e.g.:

Siirsimme pöydän ikkunan alle.
We moved the table under the window.

Could you also say Pöytä siirrettiin ikkunan alle? How is that different from Pöytä on siirretty ikkunan alle?

Yes, Pöytä siirrettiin ikkunan alle is correct. The difference is mainly tense and aspect:

Pöytä siirrettiin ikkunan alle.
- simple past passive
- The table was moved under the window (at some time in the past).
- Often used with a time expression:
  Pöytä siirrettiin ikkunan alle eilen.
  The table was moved under the window yesterday.
Pöytä on siirretty ikkunan alle.
- perfect passive
- The table has been moved under the window (and is now there / the result is relevant now).

So the perfect (on siirretty) connects the past action with the present state, while the simple past (siirrettiin) just reports a past event.

How would you say “We moved the table under the window” in Finnish using an active sentence?

A natural active sentence is:

Siirsimme pöydän ikkunan alle.

Breakdown:

siirsimme = we moved
- verb siirtää (to move something)
- past tense stem siirsi-
- personal ending -mme = we
pöydän = the table (object in accusative/genitive form)
ikkunan alle = under the window (to under the window)

So:

Siirsimme pöydän ikkunan alle.
= We moved the table under the window.

Can the word order change, for example to Ikkunan alle on siirretty pöytä? Does that change the meaning?

Yes, the word order can change, and it slightly changes the focus or information structure.

Pöytä on siirretty ikkunan alle.

Neutral word order.
pöytä is known/important information.
Often understood as “the table has been moved under the window” (that particular table everyone knows about).

Ikkunan alle on siirretty pöytä.

Emphasis is first on the location: to under the window.
pöytä appears later and may be interpreted as new information:
A table has been moved under the window.

So the basic meaning is the same (a table ends up under a window), but:

Pöytä on siirretty ikkunan alle. → focuses on the table
Ikkunan alle on siirretty pöytä. → focuses on the place and introduces a table there

This kind of word-order–based nuance is one way Finnish compensates for not having articles (a/the).

How is siirretty formed from siirtää? Is there a pattern I can use for other verbs?

Siirretty is the past passive participle of siirtää.

For a typical type 1 verb like siirtää:

Dictionary form: siirtää (to move something)
Passive present: siirretään (is/are moved)
Past passive: siirrettiin (was/were moved)
Past passive participle: siirretty (moved)

General pattern for many type 1 verbs:

rakentaa → rakennetaan → rakennettiin → rakennettu (to build → is built → was built → built)
pestä → pestään → pestiin → pesty (to wash → is washed → was washed → washed)

You use this participle with olla to form the perfect passive:

Talo on rakennettu. = The house has been built.
Auto on pesty. = The car has been washed.
Pöytä on siirretty. = The table has been moved.