Learn Hardcore Finnish: Kaupungin tavaratalo on joulukuussa hyvin ruuhkainen. - The department store in the city is very crowded in December.

kaupunki → stem kaupung-
- genitive ending -in → kaupungin
The genitive is used for possession or a close relationship, so kaupungin tavaratalo literally means the city’s department store or the department store of the city.

So -in here marks “of the city” / “city’s”.

Does kaupungin tavaratalo mean “the city’s department store” or “a department store in the city”?

It most naturally means “the city’s department store”, suggesting a specific department store associated with that city (often the main one).

However:

Finnish has no articles (a / the), so context decides whether you understand it as the or a.
kaupungin tavaratalo focuses on the possessor/owner: the city has this department store.
If you want to emphasise the location “in the city” instead, you would usually say:
- Tavaratalo on kaupungissa. – The department store is in the city.

In this sentence, kaupungin tavaratalo is best read as “the city’s department store.”

Why is tavaratalo one word and not two separate words?

Tavaratalo is a compound noun:

tavara = goods, items, stuff
talo = house, building
tavaratalo = literally “goods-house” → department store

In Finnish, compounds are almost always written as one word, not separated:

kirjakauppa (bookshop), ruokakauppa (grocery store), elokuvateatteri (cinema)

So tavara talo as two words would be incorrect; it must be tavaratalo.

Why is it joulukuussa and not just joulukuu?

The basic word is joulukuu (December). In the sentence we need the meaning “in December”, so Finnish uses a case ending:

joulukuu → stem joulukuu-
- -ssa (inessive “in”) → joulukuussa

The inessive case (-ssa / -ssä) often expresses:

physical location: talossa (in the house)
time periods: kesällä (in summer), yöllä (at night), joulukuussa (in December)

So joulukuussa literally means “in December (inside the time period of December).”

Can the word order be changed, for example to Joulukuussa kaupungin tavaratalo on hyvin ruuhkainen?

Yes, that word order is perfectly correct Finnish.

Kaupungin tavaratalo on joulukuussa hyvin ruuhkainen.
Joulukuussa kaupungin tavaratalo on hyvin ruuhkainen.

Both are grammatical. The difference is in emphasis:

Original order slightly highlights “the city’s department store” as the topic.
Starting with Joulukuussa puts more focus on the time frame (December): As for December… the city’s department store is very crowded.

But in everyday speech, both are natural and mean the same thing.

What exactly is the verb on doing here?

On is the 3rd person singular present of the verb olla (to be).

olla = to be
hän on / se on / tavaratalo on = he/she/it/the department store is

In this sentence:

Kaupungin tavaratalo = subject
on = is
hyvin ruuhkainen = a predicate adjective describing the subject

So on here simply means “is”.
(It’s true that on is also used in “has” constructions, like Minulla on auto = I have a car, but this sentence is not that structure.)

What does hyvin mean here, and how is it different from other words for “very” like tosi or erittäin?

Hyvin is the adverb from hyvä (good). It has two main uses:

“well”:
- Hän puhuu hyvin suomea. – He/She speaks Finnish well.
“very” before adjectives and some adverbs:
- hyvin ruuhkainen – very crowded
- hyvin kallis – very expensive

Compared to other intensifiers:

tosi ruuhkainen – very / really crowded (informal)
erittäin ruuhkainen – extremely / very crowded (a bit more formal/stronger)
todella ruuhkainen – really, truly crowded (also quite strong)

In this sentence, hyvin ruuhkainen is a neutral, common way to say “very crowded.”

What kind of word is ruuhkainen, and how is it formed?

Ruuhkainen is an adjective.

It is formed like this:

ruuhka = traffic jam, congestion, rush, crowding
suffix -inen → typically forms adjectives meaning “having X” / “full of X”
ruuhka + inen → ruuhkainen ≈ “full of congestion/crowds” → crowded / congested

In the sentence, it acts as a predicative adjective describing the subject:

Kaupungin tavaratalo (subject)
on (verb “is”)
hyvin ruuhkainen (what it is like)

So it matches the subject in number (singular) and is in the basic (nominative) form here.

Why is tavaratalo in the basic form, not in -ssa like joulukuussa?

Because tavaratalo is the subject of the sentence, not an adverb of place or time.

Structure:

Kaupungin tavaratalo – subject (the city’s department store)
on – verb (is)
joulukuussa – adverbial of time (in December)
hyvin ruuhkainen – predicate adjective (very crowded)

Subjects in Finnish usually appear in the nominative (basic) form:

Tavaratalo on iso. – The department store is big.
Kaupungin tavaratalo on hyvin ruuhkainen.

If you wanted to say “in the city’s department store”, then tavaratalo would take -ssa:

Kaupungin tavaratalossa on joulukuussa paljon ihmisiä.
– In the city’s department store there are many people in December.

How would I say “Department stores in the city are very crowded in December” (plural)?

A natural plural version is:

Kaupungin tavaratalot ovat joulukuussa hyvin ruuhkaisia.

Breakdown:

Kaupungin – of the city / the city’s (genitive singular)
tavaratalot – department stores (plural nominative of tavaratalo)
ovat – are (3rd person plural of olla)
joulukuussa – in December (inessive case)
hyvin ruuhkaisia – very crowded (adjective ruuhkainen in partitive plural; common when describing a general property of several things)

You could also sometimes hear Kaupungin tavaratalot ovat joulukuussa hyvin ruuhkaiset, but …ruuhkaisia is very typical and sounds natural in this kind of general statement.