Learn Hardcore Finnish: Olohuone on jo koristeltu, ja joulukuusi on tuotu sisään. - The living room has already been decorated, and the Christmas tree has been brought inside.

Olohuone on jo koristeltu.
= The decorating of the living room is finished; the result is visible now.
(focus on the current state that comes from a past action)

Why do we use passive koristeltu here? Could we say Olohuone on koristellut?

You cannot say Olohuone on koristellut here.

koristellut is active past participle (someone has decorated something).
olohuone is a place, not someone who decorates.

Olohuone on koristellut would literally mean “The living room has decorated …”, which is nonsense because it would need an object (what did the living room decorate?).

We use the passive:

Olohuone on koristeltu.
= “The living room has been decorated.”
We care about the result, not about who did it. The doer is left out.

Is koristeltu functioning as a verb or as an adjective here?

Formally, koristeltu is a passive past participle (a verb-derived form).
Functionally, in this sentence it behaves like an adjectival predicate:

It describes the current state of the subject olohuone.
It’s formed from the verb koristella, so it still keeps a strong verbal flavor (it implies a completed decorating action), but syntactically it fills the slot of “adjective-like word after olla.”

You can compare:

Olohuone on siisti. – The living room is tidy. (plain adjective)
Olohuone on koristeltu. – The living room is decorated (as a result of someone decorating it).

So: grammatically a participle, functionally very close to an adjective of state.

What is on tuotu sisään grammatically? Why tuotu and not tuodaan or tuotiin?

On tuotu is again the passive perfect:

on = from olla (to be / to have)
tuotu = passive past participle of tuoda (to bring)

It means “has been brought” (and is now here).

Comparisons:

tuodaan – present passive: “is being brought / is brought (generally)”
- Joulukuusi tuodaan sisään. – The Christmas tree is (now) being brought in / is brought in.
tuotiin – past passive: “was brought” (at some time in the past)
- Joulukuusi tuotiin sisään. – The Christmas tree was brought in.
on tuotu – perfect passive: “has been brought” (result is relevant now)
- Joulukuusi on tuotu sisään. – The tree has been brought in (and is already inside now).

The perfect fits the context of things already prepared / done.

What does sisään literally mean, and how is it different from sisällä and sisälle?

All three are related to inside, but they express different spatial ideas:

sisään – direction into (movement across the boundary)
- used with verbs of motion
- Joulukuusi on tuotu sisään. – The tree has been brought in(wards), from outside to inside.
sisälle – also direction into, often a bit more concrete / “into the interior”
- Tule sisälle. – Come inside.
  In many contexts sisään and sisälle are interchangeable; sisään is very common.
sisällä – location inside (no movement)
- Joulukuusi on sisällä. – The Christmas tree is inside.

In the sentence you gave, something is being brought in, so the directional form sisään is the natural choice.

Why is there a comma before ja here? In English we might not put a comma before and.

In Finnish, you usually put a comma between two independent main clauses, even if they are joined by ja (and).

Olohuone on jo koristeltu – complete main clause.
joulukuusi on tuotu sisään – another complete main clause.

So we write:

Olohuone on jo koristeltu, ja joulukuusi on tuotu sisään.

If the second part were not a full clause (for example, just another verb phrase sharing the same subject), then you would not put a comma:

Olohuone on jo koristeltu ja siivottu.
(Same subject olohuone, two predicates, no comma.)

Why doesn’t Finnish use any articles like the before olohuone and joulukuusi?

Finnish has no articles at all—no the, no a/an.

Whether English would use the or a/an has to be understood from:

context (what has been referred to earlier),
word order,
and sometimes case forms or other modifiers.

In this sentence:

olohuone naturally corresponds to “the living room” (probably the one both speakers know about, e.g. in a house).
joulukuusi corresponds to “the Christmas tree” (the specific one that has been brought in).

Finnish leaves “definiteness” and “indefiniteness” implicit; it’s not marked morphologically.

What are the basic forms and meanings of olohuone and joulukuusi? Are they compound words?

Yes, both are common compounds:

olohuone
- from olo (state, condition; also used in oleskella “to lounge / to hang around”)
- - huone (room)
    → literally something like “being room / lounging room”, i.e. living room.
joulukuusi
- from joulu (Christmas)
- - kuusi (spruce tree)
    → literally “Christmas spruce”, i.e. Christmas tree.

Both in the sentence are in nominative singular and act as the subjects of their clauses.

Can this sentence also imply “the living room has already been decorated by us”? How would I say who did the decorating?

Yes, Finnish passive often leaves the agent unspecified; it can easily include “we” if that makes sense from context.

To make the agent explicit, you have a few options:

Use an active form with a personal subject:
- Olemme jo koristelleet olohuoneen.
  = We have already decorated the living room.
- Olemme jo tuoneet joulukuusen sisään.
  = We have already brought the Christmas tree in.
Use a passive with an agent construction, though in everyday speech this is less common here:
- Olohuone on jo koristeltu meidän toimesta.
  = The living room has already been decorated by us.

The natural, everyday choice when you want to say “we did it” is the first: Olemme jo koristelleet olohuoneen.

Could I also say Olohuone on jo koristeltu ja joulukuusi tuotu sisään without repeating on?

Yes, that is grammatical and natural:

Olohuone on jo koristeltu ja joulukuusi tuotu sisään.

In Finnish, you can often omit a repeated verb in the second clause when it is identical to the one in the first clause, especially with olla.

Meaning-wise it’s the same. The version with on repeated:

… ja joulukuusi on tuotu sisään

can feel a bit more balanced and explicit, but both are fine in normal speech and writing.

Why are olohuone and joulukuusi in the nominative case? I thought Finnish marks subjects and objects with endings.

Finnish subjects in their basic form are usually in nominative (no extra ending):

Olohuone on… – subject: olohuone, nominative.
Joulukuusi on… – subject: joulukuusi, nominative.

Objects are where you often see special object cases (accusative / partitive), e.g.:

Koristelemme olohuoneen. – We decorate the living room (object in accusative-like form -n).

But in your sentence, both olohuone and joulukuusi are subjects of their clauses, not objects, so they stay in plain nominative.

Could I change the word order to Joulukuusi on tuotu jo sisään or Sisään on jo tuotu joulukuusi? Does the meaning change?

All of these are possible, and the core meaning stays the same, but the focus shifts.

Joulukuusi on tuotu jo sisään.
- Very natural.
- Slight emphasis that the tree itself is already inside (maybe in contrast to something else not yet done).
Sisään on jo tuotu joulukuusi.
- Starts with sisään, so the sentence foregrounds the location / direction: “Inside, already, has been brought the Christmas tree.”
- This is more marked, used when you’re emphasizing what has been brought inside (perhaps as part of a list of things being moved).
Original: Joulukuusi on tuotu sisään.
- Neutral, straightforward: joulukuusi as topic, tuotu sisään as what has happened to it.

Finnish word order is quite flexible, but changes in order are mostly about information structure (what is topic, what is new or emphasized), rather than about basic grammatical roles.

Why is jo placed before koristeltu and after on? Could it go somewhere else?

In Olohuone on jo koristeltu, the placement

on jo koristeltu

is the most neutral and common. jo (already) typically sits:

between olla (on) and the participle/verb, or
directly before the word it semantically modifies.

Other possibilities:

Olohuone jo on koristeltu. – possible, but marked; can emphasize already more, maybe as a contrast: “The living room is already decorated (even if something else isn’t).”
Olohuone on koristeltu jo. – also possible, but sounds a bit more colloquial / with a “tagged on” feel: “The living room has been decorated already, you know.”

So yes, you can move jo, but its default, neutral place here is exactly as in the original: on jo koristeltu and on tuotu jo sisään (if you wanted jo there).