Learn Hardcore Finnish: Vanhemmat nukkuvat vierashuoneessa olohuoneen vieressä. - The parents sleep in the guest room next to the living room.

So the -t here just marks that the subject is plural.
Note that vanhempi can also mean “older” as an adjective, and vanhemmat can mean “older ones”, but in this sentence it clearly means “parents.”

Why is the verb nukkuvat and not nukkuu?

In Finnish, the verb must agree with the subject in person and number.

The verb stem is nukku- (to sleep).
Present tense endings:

hän nukkuu = he/she sleeps
he nukkuvat = they sleep

Since vanhemmat is plural (parents = they), the verb needs the 3rd person plural ending -vat:

nukkuvat = (they) sleep

So: Vanhemmat nukkuvat = The parents sleep / are sleeping.

What does the ending -ssa in vierashuoneessa mean?

The ending -ssa/-ssä is the inessive case, which usually corresponds to “in” in English.

vierashuone = guest room
vierashuoneessa = in the guest room

So the idea of “in” is not a separate word; it’s built into the noun with the case ending -ssa.
There is no article “the” either; Finnish doesn’t have articles. Vierashuoneessa can mean “in a guest room” or “in the guest room”, depending on context.

Where is the English “the” in this sentence?

Finnish has no articles (no “a” or “the”). Definiteness is understood from context, word order, and what is already known in the conversation.

So:

vanhemmat can mean parents or the parents,
vierashuoneessa can mean in a guest room or in the guest room,
olohuoneen can mean of a living room or of the living room.

In most natural English translations, you’ll add “the” where it sounds right, even though Finnish doesn’t mark it explicitly.

Why does olohuoneen end in -n?

Olohuoneen is in the genitive case, which often corresponds to “of X” or shows possession or relation.

olohuone = living room
olohuoneen = of the living room / the living room’s

The postposition vieressä (“next to”) normally takes its complement in the genitive:

olohuoneen vieressä = next to the living room

So olohuoneen is genitive because it belongs with vieressä.

What exactly is vieressä, and why doesn’t Finnish just use a word like “next to” before the noun?

Vieressä is a postposition, not a preposition, and it literally comes from the noun vieri (“side”).
It is in the inessive form:

vieri = side
vieressä = at/in the side (of) → next to / beside

Postpositions usually follow the word they relate to, and that word is typically in the genitive:

olohuoneen vieressä
- olohuoneen = of the living room (genitive)
- vieressä = beside / next to

So the Finnish structure is more like “at the side of the living room”, which we translate as “next to the living room.”

Why is it vierashuoneessa olohuoneen vieressä and not something like olohuoneen vierashuoneessa?

The sentence has two separate location expressions:

vierashuoneessa = in the guest room
olohuoneen vieressä = next to the living room

They describe where the parents are, and the second one adds more detail about the position of the guest room.

You could say:

Vanhemmat nukkuvat vierashuoneessa, olohuoneen vieressä.

The comma makes it clearer that olohuoneen vieressä describes the guest room’s location.
You generally wouldn’t fuse them into olohuoneen vierashuoneessa, because that would suggest something like “in the living room’s guest room” (a genitive compound) rather than “in the guest room which is next to the living room.”

Can I change the word order, for example: Vanhemmat nukkuvat olohuoneen vieressä vierashuoneessa?

That word order is grammatically possible, but it sounds less natural. It temporarily groups olohuoneen vieressä more tightly with nukkuvat, as if the parents were next to the living room, and only then adds where exactly they are.

More natural, neutral options are:

Vanhemmat nukkuvat vierashuoneessa olohuoneen vieressä.
Vanhemmat nukkuvat vierashuoneessa, olohuoneen vieressä.
Vierashuoneessa, olohuoneen vieressä, vanhemmat nukkuvat. (emphasis on location)

Finnish word order is flexible, but changing it can affect emphasis and sometimes clarity.

Why isn’t it vierashuoneen (genitive) instead of vierashuoneessa?

Vierashuoneessa is in the inessive case and answers “where?” = in the guest room.

vierashuone (nominative) = guest room
vierashuoneessa (inessive) = in the guest room

If you used vierashuoneen (genitive), it would mean “of the guest room” or “the guest room’s,” which would need something after it:

vierashuoneen ovi = the guest room’s door
vierashuoneen vieressä = next to the guest room

So vierashuoneessa is correct here because the meaning is “in the guest room,” not “of the guest room.”

What’s the structure inside the compound word vierashuone?

Vierashuone is a compound noun:

vieras = guest
huone = room

When combined, the -s of vieras turns into -sh- in spelling: vieras + huone → vierashuone.
This is a regular pattern: s + h across a compound often becomes sh in writing (though it is pronounced as a sequence, not as English “sh”).

Then you add the case ending:

vierashuone = guest room
vierashuoneessa = in the guest room

How would the sentence change if there were several guest rooms?

You’d use the plural inessive for vierashuone:

vierashuoneissa = in (the) guest rooms

So you could say:

Vanhemmat nukkuvat vierashuoneissa olohuoneen vieressä.
= The parents sleep in the guest rooms next to the living room.

Everything else stays the same. The only change is -ssa → -issa to mark the plural inessive.

Could I say Vanhemmat nukkuvat viereisessä huoneessa instead? What’s the difference?

Yes, you can, but the meaning changes slightly:

Viereinen huone = the room next to (some reference point)
viereisessä huoneessa = in the room next to (it / something)

So:

Vanhemmat nukkuvat viereisessä huoneessa.
= The parents sleep in the room next door / in the room next to it.

Your original sentence:

Vanhemmat nukkuvat vierashuoneessa olohuoneen vieressä.

specifically says that:

It is a guest room, and
That guest room is next to the living room.

Viereisessä huoneessa is more generic: “in the room next door,” without saying it’s a guest room or specifying what it’s next to (that must be understood from context).