Learn Hardcore Finnish: Yläkerta on hiljainen illalla.

It usually refers to the upstairs part of a building, not directly to “upstairs neighbors” as people (for that you would normally say yläkerran naapurit – the neighbors upstairs).

Why is it yläkerta on hiljainen and not yläkerra or yläkerrassa?

Yläkerta is in the basic (nominative) form, because here it is the subject of the sentence:
- Yläkerta on… – The upstairs is…
Yläkerrassa (inessive case, in the upstairs) would turn the sentence into a different structure:
- Yläkerrassa on hiljaista. – It is quiet upstairs / There is quietness upstairs.

So:

Yläkerta on hiljainen.
- Focus: The upstairs (as a place) is quiet (as a characteristic, at that time).
Yläkerrassa on hiljaista.
- Focus: There is quietness in the upstairs area.

Both are grammatically correct but have different structure and nuance. The original is more like describing the state of that part of the house.

Why is it hiljainen and not hiljaista?

Both forms can appear with olla (to be), but they have different roles:

hiljainen – nominative singular adjective
- Used as a predicate adjective which directly describes the subject.
- Yläkerta on hiljainen. – The upstairs is quiet.
hiljaista – partitive singular of hiljainen
- Often used with existential sentences or to talk about an amount or presence of a quality:
- Yläkerrassa on hiljaista. – It is quiet upstairs (literally: There is quietness upstairs.)

In your sentence, hiljainen is correct because Yläkerta is treated as something that “is” quiet, not as a place where quietness exists.

So:

Yläkerta on hiljainen illalla. – describing the upstairs as quiet.
Yläkerrassa on hiljaista illalla. – describing the upstairs as a place where quietness is present.

Why is it illalla for “in the evening”? What form is that?

Illalla is the adessive singular form of ilta (evening).

Pattern:

ilta → illalla – in the evening / at evening
aamu → aamulla – in the morning
päivä → päivällä – in the daytime / during the day
yö → yöllä – at night

In Finnish, time expressions like these often use adessive (-lla/-llä) to mean “at that time”.

So:

Yläkerta on hiljainen illalla.
→ literally “The upstairs is quiet on the evening”, idiomatically “in the evening”.

Could the word order change, for example Illalla yläkerta on hiljainen?

Yes, Finnish word order is flexible, and both are correct:

Yläkerta on hiljainen illalla.
- Neutral, basic word order.
- You first introduce what you are talking about, then its state, then when.
Illalla yläkerta on hiljainen.
- Puts emphasis on illalla – In the evening, the upstairs is quiet.
- Useful if you are contrasting different times:
  - Päivällä yläkerta on meluisa, mutta illalla yläkerta on hiljainen.
    – During the day the upstairs is noisy, but in the evening the upstairs is quiet.

Both are natural; the difference is largely one of emphasis / focus, not grammar.

Why doesn’t the Finnish sentence have “the”? How do I know it’s “the upstairs”, not “an upstairs”?

Finnish has no articles (no “a/an” or “the”).

Whether you translate yläkerta as “the upstairs” or “an upstairs” is decided by context in English, not by any specific word in Finnish.

In most real situations, you are talking about a specific building you’re both aware of, so in English it becomes “the upstairs”:

Yläkerta on hiljainen illalla.
→ In a house you both know: The upstairs is quiet in the evening.
→ In a more abstract description of houses in general: Upstairs is quiet in the evening.

So: don’t look for an article in Finnish – it simply isn’t there.

What exactly does on mean here? Is it the same as the dictionary verb olla?

Yes. On is the 3rd person singular present tense form of olla (to be).

Conjugation (present tense):

minä olen – I am
sinä olet – you are (singular)
hän on – he/she is
me olemme – we are
te olette – you are (plural / formal)
he ovat – they are

In Yläkerta on hiljainen illalla,

on corresponds to “is”:
- The upstairs *is quiet in the evening.*

Could I say Yläkerrassa on hiljainen illalla?

No, that is not idiomatic Finnish.

You have two good options, but they must be structured correctly:

Yläkerta on hiljainen illalla.
- Subject in nominative (yläkerta), predicate adjective in nominative (hiljainen).
Yläkerrassa on hiljaista illalla.
- Location in inessive (yläkerrassa – in the upstairs),
- Verb on,
- Predicate in partitive (hiljaista – quietness).

Mixing yläkerrassa (inessive) with hiljainen (nominative) in this structure doesn’t work. You either:

Describe the place itself: Yläkerta on hiljainen,
or
Say there is quietness in that place: Yläkerrassa on hiljaista.

How do you pronounce yläkerta, especially the ä?

Pronunciation breakdown: ylä·ker·ta

y – like the French u in lune or German ü in München.
ä – like “a” in cat or bad, but a bit tenser and more fronted.
e – like “e” in bed.
r – trilled or tapped r, similar to Spanish r in pero.
t – like English t, but a bit “cleaner” (no strong puff of air).

Stress is on the first syllable: YLA-kerta.

Hiljainen: hil·jai·nen

h – like English h
lj – like l
- y-sound together (similar to Italian gli but lighter)
Stress also on the first syllable: HIL-jainen.

Can yläkerta refer to people, like “the upstairs neighbors are quiet”?

By itself, yläkerta refers to the upstairs floor / area, not directly to the people.

To talk about people upstairs, you’d say:

Yläkerran naapurit ovat hiljaisia. – The upstairs neighbors are quiet.
Yläkerrassa asuvat naapurit ovat hiljaisia. – The neighbors who live upstairs are quiet.

Your sentence Yläkerta on hiljainen illalla is about the place / floor, not specifically the people, even though in real life their behavior is usually the reason it’s quiet.

Could I also say something like Illalla yläkerta on yleensä hiljainen? What does yleensä add?

Yes, that’s perfectly natural:

Illalla yläkerta on yleensä hiljainen.
– In the evening, the upstairs is usually quiet.

Yleensä means “usually / generally / as a rule”.
Adding it changes the meaning from a strong statement of fact (“is quiet”) to a habitual / typical tendency (“is usually quiet, but not always”).