Learn Hardcore Finnish: Kun aurinko laskee, taivas muuttuu tummaksi.

Finnish normally uses a comma to separate a subordinate clause from the main clause. Here, Kun aurinko laskee (When the sun sets) is a subordinate clause introduced by kun, so it’s followed by a comma before the main clause taivas muuttuu tummaksi.

What does kun do here, and can it mean something other than when?

Kun is a conjunction that introduces a subordinate clause. In this sentence it means when (time).
Depending on context, kun can also mean since/because (especially in spoken Finnish), but with a clear time meaning like aurinko laskee, the natural reading is temporal: when.

Why is it aurinko laskee (present tense) instead of a past tense?

Finnish often uses the present tense for general truths, habitual events, or things that happen repeatedly:

Kun aurinko laskee, taivas muuttuu tummaksi. = Whenever/when the sun sets, the sky becomes dark.
If you mean a specific past event, you’d typically use past tense:
Kun aurinko laski, taivas muuttui tummaksi. = When the sun set, the sky became dark.

Is aurinko the subject, and why isn’t it marked in any special way?

Yes, aurinko (the sun) is the subject of laskee (sets). In Finnish, the subject is often in the basic dictionary form (nominative) with no ending, especially in simple affirmative sentences.

What’s the difference between laskea and laskemaan/lasku etc.? What exactly is laskee?

laskee is the 3rd person singular present form of the verb laskea.
laskea has several meanings (like to count, to lower, and for the sun to set). Context decides the meaning, and with aurinko, laskea is understood as to set.

Why is it taivas muuttuu and not taivas muuttaa?

muuttua means to change / to become (intransitive: the subject changes by itself).
muuttaa often means to change something (transitive: you change something) or to move/relocate.
So taivas muuttuu = the sky becomes/turns (dark), which matches the meaning.

What case is tummaksi, and why is it not just tumma?

tummaksi is in the translative case (ending -ksi), which commonly expresses a change of state: become X.
So:

taivas muuttuu tummaksi = the sky becomes dark
If you said taivas on tumma, that would mean the sky is dark (a state, not a change).

Could I also say taivas tummuu? How would that differ?

Yes. taivas tummuu means the sky darkens (it becomes dark). It uses a verb derived from the adjective.
Difference in feel:

taivas muuttuu tummaksi = explicitly describes a change into the state dark (a bit more neutral/explicit).
taivas tummuu = more compact and very natural, focusing on the process of darkening.

Does Finnish require the (definite articles) anywhere in this sentence?

No. Finnish has no articles like a/an/the. Definiteness is inferred from context. aurinko here is naturally interpreted as the sun (there’s usually only one relevant sun), and taivas as the sky.

Can the word order be changed, like Taivas muuttuu tummaksi, kun aurinko laskee?

Yes. Both are correct.

Kun aurinko laskee, taivas muuttuu tummaksi. (subordinate clause first) often sets the time frame first.
Taivas muuttuu tummaksi, kun aurinko laskee. puts the main statement first, then adds the condition/time.

How do I know which verb belongs to which clause?

Each clause has its own finite verb:

Subordinate clause: Kun aurinko laskee → verb laskee
Main clause: taivas muuttuu tummaksi → verb muuttuu
The comma is a helpful visual cue that a clause boundary is there.

How is this sentence pronounced (roughly), especially muuttuu and tummaksi?

Key points:

Finnish stress is usually on the first syllable: AU-rin-ko, TAI-vas, MUUT-tuu, TUM-ma-ksi.
muuttuu has a long uu sound and a doubled tt (held a bit longer): muut-too (with a clear long vowel).
tummaksi ends with -ksi, which is pronounced as a cluster: …mak-si.