Learn Hardcore Finnish: Paksu sumu ja tumma taivas tekevät kaupungista hiljaisen aamulla. - Thick fog and a dark sky make the city quiet in the morning.

One thing: Paksu sumu tekee… – tekee (3rd person singular)
Two things joined by ja: Paksu sumu ja tumma taivas tekevät… – tekevät (3rd person plural)

So tekevät is plural because there are two separate things doing the action.

What case is kaupungista, and why is the -sta ending used here?

kaupungista is in the elative case (ending -sta / -stä), which basically means from (inside) something.

Literally, kaupungista = from the city / out of the city.

With the verb tehdä (to make), there is a very common pattern:

tehdä jostakin jotakin
to make something out of / from something

Examples:

Tein vanhoista farkuista hameen.
I made a skirt out of old jeans.
He tekivät autiosta talosta kodikkaan.
They made a cozy place out of an abandoned house.

In the sentence:

tekevät kaupungista hiljaisen
literally: make (out) of the city (something) quiet

So kaupungista marks the source that is being transformed: “what is being made into something else.”

What is hiljaisen, and why isn’t it hiljainen or hiljaiseksi?

hiljaisen is an adjective form of hiljainen (quiet). Here it is in the genitive/accusative singular and works as the resulting state that the city ends up in.

The structure is:

tehdä
- [elative]
  - [new thing / new quality]

So:

kaupungista (from the city)
hiljaisen (a quiet one / quiet)

You can think of it as:

to make a city into a quiet (one)

Why not hiljainen?
The plain nominative hiljainen would usually be used with olla (to be):

Kaupunki on hiljainen. – The city is quiet.

But here we are not just describing the city; we’re describing a change caused by something. That’s why hiljainen takes a form that behaves like an object/result, not like a simple predicate.

Why not hiljaiseksi?
hiljaiseksi is the translative case, often used for “becoming/turning into”:

Kaupunki muuttuu hiljaiseksi.
The city becomes quiet.
He tekivät kaupungin hiljaiseksi.
They made the city quiet.

So there are two slightly different but both possible patterns:

tehdä kaupungista hiljaisen
– “make out of the city a quiet (one)” (source in elative, result as object-like)
tehdä kaupungin hiljaiseksi
– “make the city quiet” (city as direct object, result in translative)

Your sentence uses pattern 1. Pattern 2 would also be grammatical, with a slightly different structure.

Is hiljaisen grammatically connected to kaupungista, or is it a separate object?

Logically, hiljaisen describes kaupunki (the city), but grammatically it behaves like the resulting object of tehdä.

You can see kaupungista hiljaisen as a pair:

kaupungista = from the city (source)
hiljaisen = a quiet (one) (result)

Together they answer “What do the fog and sky make (out) of the city?” → They make it quiet.

So:

Semantically: hiljainen is a quality of the city.
Formally: hiljaisen is in the same kind of form an object/result would take in this construction with tehdä.

What case is aamulla, and why is it used for “in the morning”?

aamulla is in the adessive case (ending -lla / -llä). The basic meanings of adessive are:

“on / at” a surface or location → pöydällä on the table
“at” a time → kello kolmelta at three o’clock

With times of day, adessive often means “at / in [that time]”:

aamulla – in the morning
yöllä – at night
kesällä – in (the) summer
talvella – in (the) winter

Some useful contrasts:

aamulla – in the morning (one particular morning or “that” morning)
aamuisin – in the mornings / usually in the morning (habit)
aamuna – as a morning (essive; more like “on a certain morning, in the role/character of a morning”, used in special stylistic/poetic contexts)

So aamulla is the normal, neutral way to say “in the morning” here.

Could the word order change, for example to Aamulla paksu sumu ja tumma taivas tekevät kaupungista hiljaisen?

Yes, that word order is perfectly fine.

Finnish word order is relatively flexible, and you often move elements to the beginning of the sentence to highlight them.

Paksu sumu ja tumma taivas tekevät kaupungista hiljaisen aamulla.
Neutral focus on the causers (fog and sky). The time aamulla is just extra information at the end.
Aamulla paksu sumu ja tumma taivas tekevät kaupungista hiljaisen.
Now aamulla is highlighted: In the morning, (as opposed to some other time) the thick fog and dark sky make the city quiet.

Grammatically nothing changes; it’s mostly about emphasis and style.

Why are paksu and tumma both singular, even though together they form a plural subject?

Each adjective agrees with its own noun, not with the whole combined subject.

paksu sumu – thick fog (one mass of fog, singular)
tumma taivas – dark sky (one sky, singular)

Inside each noun phrase:

Adjective = singular nominative
Noun = singular nominative

Then those two noun phrases are joined by ja:

paksu sumu ja tumma taivas

As a whole, that’s a plural subject, so the verb is plural (tekevät). But each part remains grammatically singular internally; that’s why you don’t see any plural marking on paksu or tumma.

Why doesn’t Finnish use any word for “the” in paksu sumu and tumma taivas?

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

Noun phrases like:

paksu sumu
tumma taivas
kaupunki
aamu

can mean:

a thick fog / the thick fog
a dark sky / the dark sky
etc.

The difference between a and the is expressed by context, word order, and sometimes pronouns, not by special little words.

In your sentence, English naturally uses the (the thick fog, the dark sky, the city) because they are specific things in that situation, but Finnish leaves this to context and just says paksu sumu, tumma taivas, kaupungista.

What is the difference in meaning between tekevät kaupungista hiljaisen and tekevät kaupungin hiljaiseksi?

Both are possible, and both can correspond to English “make the city quiet”, but the structure is slightly different.

tekevät kaupungista hiljaisen
- kaupungista = elative “from / out of the city”
- hiljaisen = object-like result (“a quiet one”)
- Emphasizes making something out of the city – a kind of transformation:
  They make (out) of the city a quiet [place].
tekevät kaupungin hiljaiseksi
- kaupungin = genitive/accusative “(the) city” (direct object)
- hiljaiseksi = translative “into a quiet state”
- Emphasizes putting the city into a quiet state:
  They make the city quiet / turn the city quiet.

In everyday speech, pattern 2 ([object] + [adjective in -ksi]) is extremely common with verbs of causing a change:

Se uutinen teki minut surulliseksi.
That news made me sad.
Ilta-aurinko tekee huoneen lämpimäksi.
The evening sun makes the room warm.

Your sentence uses pattern 1, which leans more on the idea “what kind of thing does the city become (as a result)?” rather than “what state is the city put into?”