Learn Hardcore Norwegian: Da svaret endelig kom, kunne han slutte å sende flere e-poster. - When the answer finally arrived, he could stop sending more emails.

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.

Why is the verb kunne placed before han in the main clause (“kunne han…”)? In English we’d say “he could…”

This is the V2 (verb-second) rule in Norwegian. When an initial element (like your adverbial clause Da svaret endelig kom) occupies the first position, the finite verb must come in the second position—before the subject:
1st position: Da svaret endelig kom,
2nd position: kunne (the verb),
3rd position: han (the subject).

If the subject started the sentence, you’d say Han kunne….

What is the role of å before sende, and is it always necessary?

The particle å is the infinitive marker in Norwegian, similar to English “to.” After a verb like slutte, you use å before another verb:
• Han slutter å sende.
Exceptions (in informal speech or with modal verbs) can drop å, but in written/standard language you generally include it.

What’s the difference between slutte å sende, stoppe å sende, and holde opp med å sende? Can they all replace each other?

They all mean “to stop doing something,” but with slight nuances or different constructions:
• slutte å + verb – neutral, “cease doing something.”
• stoppe å + verb – more immediate “stop right now.”
• holde opp med å + verb – colloquial/less formal, same meaning as slutte å.

Example substitutions:
• Da svaret endelig kom, kunne han stoppe å sende flere e-poster.
• Da svaret endelig kom, kunne han holde opp med å sende flere e-poster.

What does flere e-poster mean, and why not mange e-poster?

flere means “more” (additional) rather than “many.”
• flere e-poster = more (additional) emails (beyond what he already sent).
• mange e-poster = many emails (a large total number).

Here the focus is on stopping the sending of extra messages, so flere is the right choice.

Why is e-poster hyphenated, and how do you form its plural?

Norwegian uses a hyphen when a prefix is a single letter or abbreviation:
• e-post (electronic mail).
To form the plural indefinite, you add -er:
• en e-post → flere e-poster.

Other compounds follow different rules (e.g., telefon → telefoner without a hyphen).

What’s the difference between endelig and til slutt, and why use endelig here?

Both can translate as “finally,” but:
• endelig conveys relief or impatience that something long‐awaited has happened.
• til slutt is more matter‐of‐fact: “in the end” or “eventually.”

In your sentence, endelig emphasizes relief when the answer finally came.

Can kom be replaced by ankom, and would that change the tone?

You can say ankom, but it’s more formal or literary (borrowed from “ankomme”). In everyday speech and writing, kom is by far more common:
• Da svaret endelig kom, … (neutral, standard)
• Da svaret endelig ankom, … (formal/literary)