Learn Hardcore Norwegian: Når vi ror over fjorden, må alle ha på seg flytevest. - When we row across the fjord, everyone must wear a life jacket.

når is used
- for present and future time:
  - Når vi ror over fjorden, … = When we row across the fjord (whenever/next time we do it)…
- for repeated or general actions in the past:
  - Når jeg var liten, lekte jeg ute hver dag. = When I was little, I played outside every day.
da is used
- for a single event in the past:
  - Da vi rodde over fjorden, hadde alle på seg flytevest. = When we rowed across the fjord (that time), everyone wore a life jacket.

In your sentence, this is a general rule or instruction (valid every time), so når is correct.

Why is the word order Når vi ror over fjorden and not Når ror vi over fjorden?

Når vi ror over fjorden is a subordinate clause (it cannot stand alone). In Norwegian subordinate clauses, the normal word order is:

subjunction – subject – (other elements) – verb – rest

So we get:

Når (subjunction)
vi (subject)
ror (verb)
over fjorden (rest)

Når ror vi over fjorden? would be a main clause question meaning When are we rowing across the fjord?—a different sentence type.

Why is there a comma after fjorden: Når vi ror over fjorden, må alle …?

Norwegian normally uses a comma to separate a subordinate clause from the main clause when the subordinate clause comes first.

Subordinate clause: Når vi ror over fjorden
Main clause: må alle ha på seg flytevest

Rule:
If the sentence starts with a subordinate clause, put a comma before the main clause:

Når vi ror over fjorden, må alle ha på seg flytevest.

Why does the main clause start with må and not alle? Why not … , alle må ha på seg flytevest?

Norwegian main clauses (statements) follow the V2 rule: the finite verb must be in second position.

Here, the sentence starts with a whole subordinate clause. That entire clause counts as position 1. The next element must be the finite verb of the main clause:

Når vi ror over fjorden (position 1 – whole clause)
må (finite verb in position 2)
alle (subject in position 3)
ha på seg flytevest (rest)

So:

Correct: Når vi ror over fjorden, må alle ha på seg flytevest.
Incorrect as a neutral statement: Når vi ror over fjorden, alle må ha på seg flytevest. (breaks V2)

What exactly does må mean here? How is it different from skal?

må is a modal verb expressing necessity or obligation—similar to must or have to:

må = must / have to
- Alle må ha på seg flytevest. = Everyone must wear a life jacket.

skal can express:

future: will / going to
planned / arranged action: is supposed to
obligation, but often a bit more formal or external (like a rule or order).

In this context, må is the natural choice to express a strong safety requirement. Skal would be possible but can sound a bit like citing an official rule:

Alle skal ha på seg flytevest. = Everyone shall / must wear a life jacket (according to the rules).

Why is it ha på seg for to wear? What does each word mean literally?

ha på seg is a fixed expression meaning to wear (clothes, shoes, etc.). Literally it is:

ha = have
på = on
seg = oneself (reflexive pronoun: himself/herself/themselves)

So literally: to have on oneself.

Examples with different subjects:

Jeg har på meg en jakke. = I am wearing a jacket.
Du har på deg sko. = You are wearing shoes.
Han har på seg en hatt. = He is wearing a hat.
Vi må ha på oss flytevester. = We must wear life jackets.

In the infinitive (after må) the pattern is ha på seg:

må alle ha på seg flytevest = everyone must wear a life jacket.

Why do we use seg and not dem in ha på seg?

seg is the reflexive pronoun for third person (he, she, they) and for indefinite subjects like alle (everyone).

Reflexive means that the object refers back to the same person(s) as the subject.

Alle (subject)
har på seg (have on themselves → what they themselves are wearing)

If you used dem, it would mean have on them (some other people), which doesn’t fit the meaning wear.

Compare:

Alle har på seg flytevester.
Everyone is wearing life jackets (on themselves).
Alle har på dem flytevestene de fant i båten.
Everyone has those life jackets they found in the boat on them.
(Here dem refers to the jackets as a separate object, not reflexive; this feels odd with ha på as “wear”, and native speakers would usually still use seg.)

How does ha på seg change in different persons (I, you, we, etc.)?

In the present tense, only the reflexive pronoun changes:

jeg har på meg … = I am wearing …
du har på deg … = you are wearing … (singular)
han / hun / den / det har på seg … = he / she / it is wearing …
vi har på oss … = we are wearing …
dere har på dere … = you are wearing … (plural)
de har på seg … = they are wearing …

In the infinitive after a modal like må, you use ha på + the matching reflexive for the subject:

Jeg må ha på meg …
Du må ha på deg …
Alle må ha på seg …

Why is the verb har not used after må? Why is it må alle ha på seg and not må alle har på seg?

After a modal verb in Norwegian (like må, kan, vil, skal, bør), the next verb is used in the bare infinitive, without å and without tense ending:

må
- ha (infinitive)
not må har

Examples:

Jeg må gå. (not må går)
Vi kan spise. (not kan spiser)
Du skal lese.

So:

må alle ha på seg flytevest is correct.
må alle har på seg flytevest is incorrect.

Why is it over fjorden and not something like gjennom fjorden or på fjorden?

Prepositions in Norwegian are partly about meaning and partly just conventional usage.

over fjorden = across the fjord
- You go from one side to the other (crossing). This matches rowing from one shore to the opposite shore.
på fjorden = on the fjord
- You are out on the water (in the boat), but not necessarily crossing from one side to the other.
- Vi ror på fjorden. = We are rowing on the fjord (somewhere out there).
gjennom fjorden = through the fjord
- Not idiomatic in this context; sounds odd, as if you pass through something like a tunnel.

Here, the idea is crossing the fjord, so over fjorden is the natural prepositional phrase.

Why is it fjorden (definite form) and not just fjord?

Norwegian uses the definite form of nouns (with the -en / -et / -a ending) when the thing is:

specific
known from context
or “the one we usually mean here”

fjorden = the fjord (a particular, known fjord)
en fjord = a fjord (any fjord)

In real-life use, you often talk about “the” local fjord that everybody there knows about:

Når vi ror over fjorden, … = When we row across the (local/that) fjord, …

If you said Når vi ror over en fjord, it would sound like when we row over some fjord or other, more generic and less natural in this safety-rule type sentence.

What is the verb ror from? How is å ro conjugated?

The infinitive is å ro = to row. The main forms are:

Infinitive: å ro
Present: ror
- Vi ror over fjorden. = We are rowing / we row across the fjord.
Preterite (simple past): rodde
- Vi rodde over fjorden i går. = We rowed across the fjord yesterday.
Past participle: rodd
- Vi har rodd over fjorden mange ganger. = We have rowed across the fjord many times.

So Når vi ror over fjorden uses the present tense form ror.

Can Når vi ror over fjorden refer to the future, even though it’s present tense?

Yes. In Norwegian, present tense is often used for future events when the time is clear from context:

Når vi ror over fjorden i morgen, må alle ha på seg flytevest.
= When we row across the fjord tomorrow, everyone must wear a life jacket.

This is similar to English sentences like:

We’re leaving tomorrow.
When we go there next week, …

So Når vi ror over fjorden can mean:

When we row across the fjord (whenever this happens)
or When we row across the fjord (next time / in the future)

depending on context.

What kind of word is flytevest, and how do you form its plural and definite forms?

flytevest is a compound noun:

flyte (from å flyte = to float)
vest (vest)

Together it means life jacket / buoyancy vest.

It is a masculine (often treated as en-word) noun:

Indefinite singular: en flytevest = a life jacket
Definite singular: flytevesten = the life jacket
Indefinite plural: flytevester = life jackets
Definite plural: flytevestene = the life jackets

In the sentence, flytevest is in the indefinite singular:

ha på seg flytevest = wear a life jacket (not specifically which one).