Da svaret endelig kom, kunne han slutte å sende flere e-poster.

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Breakdown of Da svaret endelig kom, kunne han slutte å sende flere e-poster.

• å: to
• kunne: can
• komme: to arrive

Questions & Answers about Da svaret endelig kom, kunne han slutte å sende flere e-poster.

What is the function of da in this sentence and when should I use da instead of når?

In Norwegian, da is a conjunction meaning “when” for a single, completed event in the past. You use da when referring to one specific moment (“Da svaret endelig kom…” = “When the answer finally arrived…”).
By contrast, når is used for:
• general truths or habitual events (Når det regner, tar jeg paraply)
• future events (Når jeg er ferdig, ringer jeg deg)

So because this is one past event (the answer’s arrival), da is correct.

Is at needed before svaret endelig kom? Why isn’t there an at?

No, you don’t add at here. Conjunctions like da, når, fordi, dersom already introduce the subordinate clause, so you omit at.
Example of an at-clause (with at):
• Hun sa at svaret endelig kom.
But with da:
• Da svaret endelig kom, kunne han slutte…

Why is there a comma after kom? Is it required?

Yes. In written Norwegian, when a sentence starts with a subordinate clause (here Da svaret endelig kom), you place a comma before the main clause. It marks the boundary between the two clauses. Omitting it isn’t a fatal error in informal text, but it’s standard to include it.