Learn Hardcore Finnish: Rappukäytävässä on hiljaista illalla. - It is quiet in the stairwell in the evening.

So rappukäytävässä literally means “in the stairwell”.
The form is -ssa or -ssä depending on vowel harmony; because rappukäytävä has the front vowel ä, the inessive ending is -ssä.

Why is it hiljaista and not hiljainen?

Hiljaista is the partitive singular of the adjective hiljainen (quiet).

In sentences like On hiljaista or Rappukäytävässä on hiljaista, Finnish is talking about a state/amount of quietness, almost like saying “there is quiet(ness)”.

This type of impersonal / existential sentence often uses the partitive for:

indefinite amounts (vettä on = there is (some) water)
ongoing or general states (on hiljaista = it is (generally) quiet)

If you said Rappukäytävä on hiljainen, that treats hiljainen as a normal predicate adjective describing the stairwell as a characteristic, more like:

Rappukäytävä on hiljainen = The stairwell is quiet (as a quality).
Rappukäytävässä on hiljaista = There is quietness in the stairwell / It is quiet in the stairwell.

Both are correct, but the structure and nuance differ.

Can I say Rappukäytävä on hiljainen illalla instead? How is it different?

Yes, you can, and it’s perfectly correct:

Rappukäytävässä on hiljaista illalla.
→ Focus on the situation in the stairwell: In the stairwell, it is quiet in the evening.
Rappukäytävä on hiljainen illalla.
→ Focus on the stairwell as having the property of being quiet in the evening: The stairwell is quiet in the evening.

In everyday speech, they are very close in meaning.
The version with rappukäytävässä on hiljaista sounds a bit more like describing what it’s like there at that time, rather than giving a stable property of the stairwell.

Why is illalla used and not ilta or illassa?

Illalla is the adessive case of ilta (evening) and it is the normal way to express “in the evening” in Finnish:

ilta = evening (basic form)
illalla = in the evening, at evening time

For times of day, Finnish typically uses the -lla / -llä ending:

aamulla = in the morning
päivällä = in the daytime / during the day
illalla = in the evening
yöllä = at night

So illalla is a standard time expression.
Illassa would mean literally “in the evening (as inside the evening)”, and is not used for normal time-of-day expressions.

What is the role of on in this sentence?

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

In this sentence:

Rappukäytävässä = in the stairwell (location)
on = there is / is
hiljaista = quietness / it is quiet
illalla = in the evening

So the structure is the typical existential construction:

[Location] + on + [something] (+ time expression)
Rappukäytävässä on hiljaista illalla.
In the stairwell, (there) is quiet(ness) in the evening.

Why isn’t there a word for “the” (as in “the stairwell”)?

Finnish has no articles like English “a / an / the”.

Whether rappukäytävässä means “in a stairwell” or “in the stairwell” is understood from context, not from a separate word.

So:

rappukäytävässä can translate as “in the stairwell” or “in a stairwell” depending on what the speaker and listener know.

Can I change the word order, for example Illalla rappukäytävässä on hiljaista?

Yes, Finnish word order is quite flexible, and you can move parts to change emphasis:

Rappukäytävässä on hiljaista illalla.
Neutral: In the stairwell, it’s quiet in the evening.
Illalla rappukäytävässä on hiljaista.
Emphasis on illalla: In the evenings (as for the time), it’s quiet in the stairwell.
Rappukäytävässä illalla on hiljaista.
Slight emphasis on rappukäytävässä illalla as a combined setting.

All are grammatically correct; the main difference is what part you want to highlight.

What exactly does rappukäytävä mean, and how is it formed?

Rappukäytävä is a compound noun:

rappu = stair, step; often used in apartment buildings to mean a stairwell/entrance section
käytävä = corridor, hallway

Combined, rappukäytävä literally is something like “stair corridor”, and it means stairwell / staircase (in a building).

Then rappukäytävässä adds the inessive case -ssä → in the stairwell.

How is rappukäytävässä pronounced?

Pronunciation tips:

Syllables: rap-pu-käy-tä-väs-sä
Stress is always on the first syllable: RAP-pu-käy-tä-väs-sä
Double consonants (pp, ss) are held longer:
- rappu: the p is lengthened
- -ssä: the s is lengthened
ä is an open front vowel (like the a in English “cat”).
y (in käy) is a front rounded vowel, a bit like trying to say “ee” with rounded lips.

Very roughly in English-like spelling: “RAHP-poo-kæy-tæ-væss-sæ”, with clear, even syllables.