Learn Hardcore Norwegian: Bussen er langsom når det er mye trafikk. - The bus is slow when there is a lot of traffic.

Buss is a masculine noun. The ending -en makes it definite: bussen = “the bus.” Norwegian often uses the definite singular to make general statements about a type of thing or a service, so Bussen er langsom... can mean “the bus (as a mode of transport/this bus service) is slow...,” not necessarily one particular physical bus.

Could I say Busser er langsomme når det er mye trafikk instead?

Yes. That means “Buses are slow when there is a lot of traffic,” a general statement about buses in plural. Both are fine:

Bussen er langsom... = generic definite (often about a service/route).
Busser er langsomme... = general plural statement about buses as a class.

Why is it langsom and not langsomme after er?

Predicate adjectives (after “to be”) generally use the indefinite form and agree with the subject:

m/f singular: langsom → Bussen er langsom.
neuter singular: langsomt → Toget er langsomt.
plural: langsomme → Bussene er langsomme. Only attributive adjectives before a definite noun take the definite form: den langsomme bussen.

Could I write Bussen er langsomt?

No. Bussen is common gender singular, so the predicate adjective must be m/f singular: langsom. The neuter form langsomt is used with neuter subjects (e.g., Toget er langsomt).

Can I say Bussen er sakte?

Not in standard Bokmål. Sakte is primarily an adverb (“slowly”), so you’d say:

Bussen går sakte (the bus goes/moves slowly). As an adjective, use langsom or treg:
Bussen er langsom / treg.

What’s the difference between langsom, treg/treig, sakte, and sen/sein?

langsom: adjective, neutral/standard for “slow” in speed.
treg (dialectal spelling treig): adjective, “sluggish/slow,” very common in speech.
sakte: adverb (“slowly”): Bussen går sakte. Comparative: saktere.
sen/sein: “late,” about time, not speed (e.g., Bussen er sen = “The bus is late,” not “slow”).

Why use når and not da, om, or hvis?

når = when/whenever (habitual, general time): ... når det er mye trafikk.
da = when (past, one-time event): Da det var mye trafikk, kom vi sent.
om/hvis = if (condition): Hvis/Om det er mye trafikk, er bussen langsom.

Is the word order inside the når-clause correct?

Yes. Subordinate clauses do not have V2. The order is subordinator + subject + verb:

når det er mye trafikk (not: ✗ når er det mye trafikk).

What happens if I put the når-clause first?

You get inversion in the main clause because of V2:

Når det er mye trafikk, er bussen langsom. Colloquially, some add så after the comma: Når det er mye trafikk, så er bussen langsom. It’s common in speech but avoided in formal writing.

Do I need a comma before the når-clause at the end?

No comma is used when the subordinate clause comes at the end:

Bussen er langsom når det er mye trafikk. If the subordinate clause comes first, you place a comma after it:
Når det er mye trafikk, er bussen langsom.

Why is it det er mye trafikk and not der er mye trafikk?

Norwegian uses the dummy subject det for existential “there is/are”:

Det er mye trafikk = “There is a lot of traffic.” Der means “there” (a place), not the existential “there.”

Why mye trafikk and not mange trafikker?

Trafikk is treated as a mass/uncountable noun in this sense, so you use mye (“much/a lot of”) with it:

mye trafikk = “a lot of traffic.” Use mange (“many”) with countable nouns:
mange biler (“many cars”).

Can I say stor trafikk for “a lot of traffic”?

That’s not idiomatic. Use:

mye trafikk (a lot of traffic),
tett trafikk or tung trafikk (heavy/dense traffic).

Are there other natural ways to say the bus is slow here?

Yes:

Bussen går sakte når det er mye trafikk.
Det går tregt for bussen når det er mye trafikk.
Bussen bruker lang tid når det er mye trafikk.
Focus on delay: Bussen blir forsinket når det er mye trafikk.

How do I say “slower/slowest” with these words?

langsom → langsommere, langsomst
treg → tregere, tregest
sakte (adverb) → saktere, saktest

Any quick pronunciation tips?

Bussen: stress on the first syllable; double s gives a long s: [BUS-sen].
langsom: [LANG-som]; the ng is a velar [ŋ].
trafikk: stress on the last syllable: tra-FIKK; final kk is a long k. In casual speech, det in når det er often sounds like [de], with a very light or silent t.