Learn Hardcore Finnish: Sytytän lampun, koska huone on edelleen pimeä. - I turn on the lamp because the room is still dark.

Sytytän is the 1st person singular present tense form of the verb sytyttää (“to light, to turn on [a light]”). Morphologically it breaks down as:

sytyt- (stem of sytyttää)
-ä- (present tense marker)
-n (1 sg ending)

So sytytän literally means “I light” or “I turn on.”

Why is lampun used here? Which case is it?

Lampun is the singular accusative (or “object”) form of lamppu (“lamp, light”). In Finnish, a direct object of a transitive verb in a completed action often takes the accusative. The accusative singular of many words coincides with the genitive, so lampun marks “the lamp” as the thing you are lighting.

Is the comma before koska necessary? When should you use a comma there?

In Finnish, you often set off a subordinate clause with a comma. Before koska (“because”) it’s recommended—though not always strictly mandatory—to insert a comma to signal the reason clause. Especially in longer sentences, the comma before koska improves readability.

Does the word order change in the clause introduced by koska?

No. After koska, Finnish subordinate clauses generally keep the same basic SVO (subject–verb–object) order as main clauses. In our example:
Subject: huone (the room)
Verb: on (is)
Adverb: edelleen (still)
Predicate adjective: pimeä (dark)

What does edelleen mean here? How is it different from vielä?

Both edelleen and vielä can mean “still.” Differences:

edelleen is slightly more formal and often emphasizes continuity.
vielä is more neutral or colloquial and can sometimes mean “yet.”

You could also say huone on vielä pimeä, but edelleen sounds a bit more formal or literary.

Can koska be replaced with siksi että?

Yes, but with a small adjustment.

koska = “because” (a reason‐introducing conjunction)
siksi että = “for that reason that” (lit. “therefore that”)

If you use siksi että, the sentence becomes:
Sytytän lampun, siksi että huone on edelleen pimeä.

That construction is slightly heavier/ more emphatic.

Could I move the reason clause before the main clause? How would I punctuate it?

Absolutely. In Finnish you can front the subordinate clause:
Koska huone on edelleen pimeä, sytytän lampun.

When the koska–clause comes first, the comma is obligatory.