Learn Hardcore Finnish: Kuusen alle laitetaan lahjapaketit vasta jouluaattona. - The gift packages are put under the tree only on Christmas Eve.

Many Finnish postpositions (words like “under”, “behind”, “in front of” that come after the noun) require the noun before them to be in the genitive.

Here, kuusen alle literally means “to under the tree”:

kuusen – of the tree
alle – to under

So you must say kuusen alle, not *kuusi alle, just like in other postpositional phrases:

talon taakse – behind the house (literally: to the house’s behind)
pöydän päälle – onto the table (to the table’s top)

What is the difference between alle and alla? Why is it kuusen alle here and not kuusen alla?

Alla and alle are related forms of the same word, but they express different things:

alla = under, at a location (static)
- kuusen alla = under the tree (the state of being there)
alle = to under, movement to a location (direction)
- kuusen alle = to under the tree (movement towards that place)

The verb laittaa (to put) expresses movement / placement, so you need the directional form alle:

Lahjapaketit ovat kuusen alla.
The gift parcels are under the tree. (static location)
Lahjapaketit laitetaan kuusen alle.
The gift parcels are put under the tree. (movement/placement)

So:

alla = where something is
alle = where something is put / moved to

Why is there no explicit subject (like “we” or “they”) with laitetaan? Who is doing the action?

Laitetaan is the impersonal / passive form of laittaa in the present tense. Finnish uses this form when:

the doer is unknown, unimportant, or general, or
you want to talk about what “people” or “we” typically do.

So Kuusen alle laitetaan lahjapaketit vasta jouluaattona means something like:

“The presents are (typically) put under the tree only on Christmas Eve,” or
“We/they only put the presents under the tree on Christmas Eve.”

In English we often translate this kind of Finnish passive as:

“we …” (if it’s about a custom or habit), or
“they … / people …”, or
with an English passive: “The presents are put…”

Grammatically, in this kind of sentence there is no explicit subject in Finnish. The verb ending -taan / -tään itself signals the impersonal/passive.

What tense is laitetaan, and does it also refer to the future?

Laitetaan is present tense, passive/impersonal.

However, Finnish often uses the present tense to talk about future events, especially when they are:

scheduled, customary, or
clearly future from the context.

So laitetaan here can be understood as:

“are put” / “are being put” (generic present), or
“will be put” (future meaning from context).

If you wanted a past version, you would say:

Kuusen alle laitettiin lahjapaketit vasta jouluaattona.
The presents were only put under the tree on Christmas Eve.

Lahjapaketit looks like a plural nominative, but it’s the object. Why isn’t there some special object ending?

You’re right that lahjapaketit is nominative plural:

lahjapaketti – a gift parcel (singular)
lahjapaketit – gift parcels (plural nominative)

In active sentences, the total object often appears in the accusative, but in the plural the accusative and nominative look the same:

(Me) laitamme lahjapaketit kuusen alle.
We put the gift parcels under the tree.

Here lahjapaketit is both nominative plural form and accusative plural function (object). The form doesn’t change.

In the passive / impersonal:

Kuusen alle laitetaan lahjapaketit.

the total object appears in the nominative (not in the genitive or special accusative), because there is no personal subject.

You see the difference clearly in the singular:

Active: (Me) laitamme lahjapaketin kuusen alle.
(accusative singular: -n)
Passive: Kuusen alle laitetaan lahjapaketti.
(nominative singular: no -n)

So:

plural total objects: same form for nominative and accusative (lahjapaketit)
in passive, the total object stays nominative, not genitive/accusative.

What exactly does vasta mean here, and how is it different from vain?

In this context, vasta means roughly “not until” or “only (later than you might expect)”. It emphasizes that something happens later than some assumed point.

So:

vasta jouluaattona ≈ not until Christmas Eve / only on Christmas Eve (and not earlier)

Compare with vain and ainoastaan:

vain = only, merely, just (quantity or limitation in general)
ainoastaan = only, solely (a bit more formal)
vasta = only when/then, not until (focus on time or stage being later)

Examples:

Söin vasta illalla.
I didn’t eat until the evening.
Söin vain yhden omenan.
I only ate one apple.

So using vasta here tells you that putting the gifts under the tree is deliberately delayed until Christmas Eve.

What case is jouluaattona, and why does it mean “on Christmas Eve”?

Jouluaattona is the essive singular of jouluaatto (Christmas Eve):

jouluaatto – Christmas Eve (base form)
jouluaattona – on Christmas Eve

The essive case (ending -na / -nä) has several uses, and one very common one is time expressions, meaning “on / during X”:

maanantaina – on Monday
kesällä is actually adessive, but: syksynä (less common) – in autumn
syntymäpäivänäni – on my birthday

So:

jouluaattona literally: “as Christmas Eve” or “in the state of Christmas Eve,”
but functionally: “on Christmas Eve.”

Could I change the word order, for example to Lahjapaketit laitetaan kuusen alle vasta jouluaattona? Does it change the meaning?

Yes, you can change the word order, and the basic meaning stays the same:

Kuusen alle laitetaan lahjapaketit vasta jouluaattona.
Lahjapaketit laitetaan kuusen alle vasta jouluaattona.

Both mean: “The gift parcels are only put under the tree on Christmas Eve.”

Word order in Finnish is often used for emphasis / focus rather than basic grammar. Roughly:

Starting with Kuusen alle emphasizes the location:
“As for under the tree – that’s when/where the presents are (only) put on Christmas Eve.”
Starting with Lahjapaketit emphasizes the presents as the topic:
“As for the gift parcels – they’re only put under the tree on Christmas Eve.”

All of these are grammatically fine (with subtle emphasis differences):

Kuusen alle laitetaan lahjapaketit vasta jouluaattona.
Lahjapaketit laitetaan kuusen alle vasta jouluaattona.
Vasta jouluaattona lahjapaketit laitetaan kuusen alle. (strong emphasis on not until Christmas Eve)

How would I say this in a normal active sentence with “we”?

You can turn the passive into a personal active sentence like this:

Me laitamme lahjapaketit kuusen alle vasta jouluaattona.
We only put the gift parcels under the tree on Christmas Eve.

Structure:

Me – we (subject)
laitamme – we put (1st person plural of laittaa)
lahjapaketit – the gift parcels (object, plural total object)
kuusen alle – under the tree (direction)
vasta jouluaattona – not until Christmas Eve

The meaning in context is basically the same as the passive version, but:

The passive sounds a bit more like a statement of general custom.
The active with “me” sounds more like we specifically as a family or group do this.

Is kuusi always “Christmas tree”, or does it just mean “spruce”? Why use kuusi here?

Kuusi primarily means “spruce” (the tree species). However, in Finnish culture the Christmas tree is usually a spruce, so in a Christmas context:

kuusi often naturally means “(Christmas) tree”.

If you want to be explicit, you can say:

joulukuusi – Christmas tree

So you might also hear:

Joulukuusen alle laitetaan lahjapaketit vasta jouluaattona.
The Christmas tree is explicitly mentioned.

But in many contexts around Christmas, kuusi alone is clearly understood as the Christmas tree.

Are there any notable sound or form changes (like stem changes) in words such as kuusi → kuusen and jouluaatto → jouluaattona?

Yes, there are some regular patterns worth noticing:

Vowel alternation / extra vowel in genitive:
- kuusi → kuusen
  The stem ends up with -e- before the -n of the genitive:
  kuus(e)n → kuusen.
  This is a common pattern for many words ending in -si:
  - vuosi → vuoden (year)
  - kuusi → kuusen (spruce)
Long vowels preserved in case forms:
- jouluaatto → jouluaattona
  The long aa in aatto stays long in the essive:
  - aamu → aamuna (morning → in the morning)
  - aatto → aattona (eve → on the eve)
Plural formation:
- lahjapaketti → lahjapaketit
  - -i plural marker + t
  - paketti → paketit (package → packages)
  - lahjapaketti → lahjapaketit (gift package → gift packages)

All of these follow regular Finnish patterns; they just take a bit of getting used to.

Can vasta move to other positions in the sentence, and does that change what is emphasized?

Yes, vasta is quite mobile, and its position affects emphasis slightly, though the core meaning stays “not until / only then”.

Some variants:

Kuusen alle laitetaan lahjapaketit vasta jouluaattona.
Neutral / common: “The gifts are (only) put under the tree on Christmas Eve (not before).”
Kuusen alle laitetaan vasta jouluaattona lahjapaketit.
Here vasta jouluaattona is closer to laitetaan:
more focus on the time of the action.
Vasta jouluaattona laitetaan lahjapaketit kuusen alle.
Strong emphasis: only on Christmas Eve, not earlier at all.

In all cases, vasta keeps its sense of “not until / only then”; moving it mainly changes which part of the sentence is highlighted as new or contrastive information.