Learn Hardcore Finnish: Uusi kirjahylly seisoo lattialampun vieressä olohuoneessa. - The new bookcase stands next to the floor lamp in the living room.

This is grammatically correct, but it sounds more neutral or slightly less vivid. Seisoo paints a clearer picture: the bookshelf is standing there.

For other objects, Finns choose verbs according to their typical position:

Matto makaa lattialla. – The rug lies on the floor.
Lamppu roikkuu katosta. – The lamp hangs from the ceiling.

Why are both uusi and kirjahylly in the same form? Why not uuden kirjahyllyn?

In this sentence, uusi kirjahylly is the subject, so it must be in the nominative singular:

uusi = new (nominative singular)
kirjahylly = bookshelf (nominative singular)

Adjectives agree with the noun in case and number:

nominative: uusi kirjahylly – a/the new bookshelf
genitive: uuden kirjahyllyn – of the new bookshelf
inessive: uudessa kirjahyllyssä – in the new bookshelf, etc.

So we say:

Uusi kirjahylly seisoo... (subject → nominative) but:
Pidän uudesta kirjahyllystä. (object of a postposition that takes elative → uudesta kirjahyllystä)

How is kirjahylly formed? Why is it one word and not kirja hylly?

Kirjahylly is a compound noun:

kirja = book
hylly = shelf
→ kirjahylly = bookshelf (literally book-shelf)

In Finnish, related nouns are very commonly written as one compound word:

ruokapöytä (ruoka + pöytä) = dining table
keittiökaappi (keittiö + kaappi) = kitchen cupboard
koulukirja (koulu + kirja) = school book

Writing kirja hylly as two words would be interpreted as a book, a shelf (two separate things), not a single piece of furniture.

What is the base form of seisoo, and how is it conjugated here?

The dictionary (infinitive) form is seisoa = to stand.

Conjugation in the present tense:

minä seison
sinä seisot
hän / se seisoo
me seisomme
te seisotte
he / ne seisovat

In the sentence:

seisoo = 3rd person singular present (he/she/it stands)

It matches the subject uusi kirjahylly (the new bookshelf → it stands).

Why is it lattialampun and not lattialamppu?

Lattialamppu = floor lamp (nominative singular).
In the sentence we have lattialampun, which is genitive singular.

Structure: [GENITIVE] + vieressä = next to [something]

lattialampun vieressä = next to the floor lamp (literally: at the side of the floor lamp)

Form:

nominative: lattialamppu
genitive: lattialampun

Notice the consonant gradation:

pp → p in the genitive: lamppu → lampun, lattialamppu → lattialampun

So the genitive is required by the postposition vieressä.

What exactly is vieressä, and why does it use the genitive lattialampun before it?

Vieressä comes from the noun vieri = side.
vieressä is the inessive form: in/at the side.

With a noun in the genitive, it works as a postposition:

lattialampun vieressä = at the side of the floor lamp → next to the floor lamp
talon vieressä = next to the house
auton vieressä = next to the car

Pattern:

[noun in genitive] + vieressä

So:

lattialamppu → lattialampun vieressä

What is the difference between vieressä and vierellä?

Both are very similar and often interchangeable; both can mean next to / beside.

vieressä – inessive (-ssa/-ssä) → literally in/at the side (of)
vierellä – adessive (-lla/-llä) → literally on/by the side (of)

Nuance:

vieressä is a bit more neutral and very common.
vierellä can sound slightly more poetic or emotional, especially with people:
- Istuin hänen vierellään. – I sat by his/her side.

In your sentence, lattialampun vieressä is the most natural choice, but lattialampun vierellä would not be wrong; it just feels a little less common in this everyday context.

Why is olohuoneessa in the form -ssa? What case is it?

Olohuoneessa is the inessive case of olohuone (living room).

base form: olohuone – living room
inessive: olohuoneessa – in the living room

The inessive -ssa / -ssä usually means in / inside:

pöydässä – in the table (e.g. a hole, drawer, leg)
kaupungissa – in the city
kaapissa – in the cupboard

So olohuoneessa clearly expresses the location in the living room.

Does olohuoneessa describe where the bookshelf is, or where the floor lamp is?

Grammatically, olohuoneessa is a separate location phrase that applies to the whole situation:

Uusi kirjahylly seisoo lattialampun vieressä olohuoneessa.

Natural interpretation: both the bookshelf and the lamp are in the living room, and the bookshelf is next to the lamp there.

You could emphasize slightly different things by changing word order:

Olohuoneessa uusi kirjahylly seisoo lattialampun vieressä.
– In the living room, the new bookshelf stands next to the floor lamp.

But in everyday usage, listeners will assume all the furniture mentioned is in the living room unless context says otherwise.

Can the word order be changed? For example, can I say Uusi kirjahylly seisoo olohuoneessa lattialampun vieressä?

Yes, Finnish word order is quite flexible. Your version is grammatical:

Uusi kirjahylly seisoo olohuoneessa lattialampun vieressä.

Both sentences mean the same in practice:

Uusi kirjahylly seisoo lattialampun vieressä olohuoneessa.
Uusi kirjahylly seisoo olohuoneessa lattialampun vieressä.

The differences are in information structure / emphasis:

Placing olohuoneessa earlier can make the location (living room) feel slightly more important or already known.
Placing lattialampun vieressä earlier can highlight the relation to the lamp a bit more.

But these are subtle; both are perfectly natural.

How do we know it means the new bookshelf and not a new bookshelf, since there is no article in Finnish?

Finnish has no articles like a / an / the. The same sentence can correspond to either a or the in English:

Uusi kirjahylly seisoo lattialampun vieressä olohuoneessa.

Could be translated as:

A new bookshelf stands next to the floor lamp in the living room.
The new bookshelf stands next to the floor lamp in the living room.

The choice between a and the depends entirely on context, not on any specific word or ending in Finnish. If the speaker and listener already know about this particular bookshelf, English needs the; if it is mentioned for the first time, English might use a.

How would the sentence change if there were several new bookshelves instead of just one?

Then the subject would be in plural nominative, and the verb would be plural too:

Uudet kirjahyllyt seisovat lattialampun vieressä olohuoneessa.

Changes:

uusi → uudet (adjective plural nominative)
kirjahylly → kirjahyllyt (noun plural nominative)
seisoo → seisovat (3rd person plural)

The rest stays the same:

lattialampun vieressä – next to the floor lamp
olohuoneessa – in the living room