Learn Hardcore Finnish: Uudenvuodenaattona kadut täyttyvät ihmisistä ja raketeista. - On New Year’s Eve the streets fill with people and fireworks.

In everyday Finnish, uuden vuoden aatto is commonly fused into the compound uudenvuodenaatto, and with the essive ending -na you get uudenvuodenaattona = on New Year’s Eve.

So:

uusi vuosi → uuden vuoden aatto → uudenvuodenaatto → uudenvuodenaattona

This kind of compounding (especially with holiday names) is very typical in Finnish.

Why is Uudenvuodenaattona capitalized? Are holiday names normally capitalized?

It is capitalized here only because it is the first word of the sentence, not because it is a holiday.

In standard Finnish:

Names of holidays (like uudenvuodenaatto, joulu, pääsiäinen) are normally written with a lowercase letter.
You only capitalize the first word of a sentence and proper names (like Suomi, Helsinki, Mikko).

So inside a sentence you’d write:

Tapaan heidät uudenvuodenaattona.
I’ll meet them on New Year’s Eve.

What case is -na in Uudenvuodenaattona, and what does it mean?

The ending -na/-nä is the essive case.

Here, aattona = as/equivalent to the day called “Eve”, and with the whole compound it means:

uudenvuodenaattona = on New Year’s Eve

The essive is often used:

for times and dates:
- maanantaina = on Monday
- jouluaattona = on Christmas Eve
for roles or states:
- opettajana = as a teacher
- lapsena = as a child / when (I was) a child

So here it’s the “on (that day)” usage: uudenvuodenaattona = (when it is) New Year’s Eve → on New Year’s Eve.

Could I also say Uuden vuoden aattona instead of Uudenvuodenaattona?

Yes, you can.

Both:

Uudenvuodenaattona
Uuden vuoden aattona

are acceptable and mean the same thing: on New Year’s Eve.

Differences:

Uudenvuodenaattona is a single compound word, more lexicalized (feels like the “official name” of the day).
Uuden vuoden aattona is more transparent to learners (you clearly see uusi vuosi
- aatto
  - -na).

In everyday speech and writing, you will see both. Style guides and dictionaries commonly list the compound form uudenvuodenaatto.

What is the grammatical role of kadut in this sentence?

Kadut is the subject of the sentence.

Base form: katu = street
Nominative plural: kadut = streets

So:

kadut täyttyvät = the streets fill up / the streets become full

Even though in English “the streets fill with people and rockets” feels a bit like the streets are locations, in Finnish kadut is grammatically the thing undergoing the change of state: they become full. That’s why it’s plain nominative plural, not a case like kaduilla or kaduille.

Why is it kadut and not something like kaduilla (on the streets)?

The verb täyttyä (to fill up, to become full) is about the subject itself becoming full. So the thing that becomes full is the subject in nominative:

katu täyttyy = the street gets full
kadut täyttyvät = the streets get full

If you said kaduilla, that would be an adessive form meaning on the streets, which would fit with some other verb, for example:

Kaduilla on paljon ihmisiä.
There are many people on the streets.

But with täyttyä, Finnish chooses:

nominative for the thing that fills (kadut),
another case (here elative) for what they fill with (ihmisistä ja raketeista).

What exactly does täyttyvät mean, and how is it different from täyttävät?

There are two related verbs:

täyttää (transitive) = to fill something
- He täyttävät kadut roskilla. = They fill the streets with trash.
täyttyä (intransitive) = to become full, to fill up
- Kadut täyttyvät ihmisistä. = The streets fill up with people.

In your sentence:

täyttyvät is the 3rd person plural present form of täyttyä:
- katu täyttyy (one street)
- kadut täyttyvät (several streets)

So the sentence literally means “On New Year’s Eve the streets become full of people and rockets.”

Why are ihmisistä and raketeista in the -sta/-stä form? Doesn’t that usually mean “from”?

Yes, -sta/-stä is the elative case, often translated as from / out of.
Here, however, the verb täyttyä uses the elative to express what something becomes full of.

Pattern:

täyttyä jostakin = to become full of something

So:

kadut täyttyvät ihmisistä ja raketeista
= literally “the streets become full from people and rockets”
→ idiomatically “the streets fill with people and rockets”.

Some other verbs or adjectives also take elative in similar “content” meanings, but täyttyä + elative is especially common.

Why is it ihmisistä and not something like ihmisillä?

Different cases answer different “with” meanings in English:

ihmisillä (adessive) usually has a “on/at/with (possessed by)” sense:
- Ihmisillä on raketteja. = People have rockets.
ihmisistä (elative) here shows what something becomes full of because of the verb täyttyä.

So:

kadut täyttyvät ihmisistä
= the streets fill up with people (become full of people)

If you used ihmisillä, it would no longer fit the verb täyttyä; it would sound like you’re talking about people having/possessing something, not the content of the filling.

How do we get from ihminen to ihmisistä? The stem seems to change.

The word ihminen (person, human) has an irregular-looking stem, but it’s very common and worth memorising.

Key forms:

nominative singular: ihminen
nominative plural: ihmiset
elative plural: ihmisistä

So the stem in most inflected forms is ihmis-, not ihminen-.

Rough pattern:

ihminen → stem ihmis- →
- ihmistä (partitive sg)
- ihmisen (genitive sg)
- ihmiset (nom pl)
- ihmisistä (elative pl)

This kind of stem change is just lexical; you mainly learn it by exposure.

What case are ihmisistä and raketeista, and what are their base forms?

They are both elative plural:

Base: ihminen → ihmisistä (from/of people)
- ihmiset = people (nom. plural)
- ihmisistä = from/of people (elative plural)
Base: raketti → raketeista (from/of rockets)
- raketit = rockets (nom. plural)
- raketeista = from/of rockets (elative plural)

Elative plural is formed by:

taking the plural stem (ihmis- → ihmis-i-, rakett- → rakett-e-),
adding -sta/-stä → -ista / -ista depending on vowel harmony.

In this sentence they mean “full of people and rockets” because of the verb täyttyä + elative.

Why is the verb plural (täyttyvät) and not singular, like täyttyy?

In Finnish, the verb normally agrees with the subject in number.

Singular subject: katu täyttyy
The street fills up.
Plural subject: kadut täyttyvät
The streets fill up.

Since the subject here is kadut (plural), the verb is täyttyvät, 3rd person plural.

Could the word order be different, for example Kadut täyttyvät uudenvuodenaattona ihmisistä ja raketeista?

Yes, Finnish word order is fairly flexible, and that alternative is possible.

Your original:

Uudenvuodenaattona kadut täyttyvät ihmisistä ja raketeista.
→ Puts strong emphasis on when this happens (on New Year’s Eve).

Alternative:

Kadut täyttyvät uudenvuodenaattona ihmisistä ja raketeista.
→ Starts with the streets as topic; the time is added later.

The core grammar (cases, verb forms) does not change; only the information structure and emphasis shift slightly. Both would be understood the same way in most contexts.

Does raketeista mean literal rockets (missiles) or fireworks?

The base word raketti can mean both rocket in the general sense and firework rocket. In the context of:

Uudenvuodenaattona kadut täyttyvät ihmisistä ja raketeista.

a Finnish reader will naturally interpret raketeista as firework rockets, because that matches the cultural context of New Year’s Eve: people in the streets, and lots of fireworks.