Learn Hardcore Norwegian: Vi må fyre i vedovnen når det er kaldt. - We have to fire up the wood stove when it is cold.

må ≈ have to / need to (very common, neutral)
It can also be translated as must, but English must often sounds a bit stronger or more formal than Norwegian må.

So må here is simply that it is necessary to light the stove when it’s cold.

What exactly does fyre mean? Is it like “to fire,” “to burn,” or “to light”?

Fyre is a verb used for burning fuel in something like a stove or fireplace.

Typical meanings:

fyre i vedovnen / peisen = to burn wood / keep a fire going in the wood stove / fireplace
Often translated as “to make a fire,” “to burn wood,” “to stoke the stove,” “to fire up the stove.”

Compared to other verbs:

tenne = to light (a candle, a match, a fire)
- tenne et bål = light a bonfire
brenne = to burn (be on fire)
- Huset brenner = The house is burning.
fyre = to operate a fire by burning fuel in a heating device
- fyre i ovnen = burn fuel in the stove

So here fyre is specifically about using the stove for heating by burning wood in it.

Why is the preposition i used in fyre i vedovnen instead of på?

I means in / inside, and på means on / on top of.

You say fyre i vedovnen because you are making a fire inside the stove:

fyre i vedovnen = burn wood in the stove (inside the firebox)

If you said på vedovnen, it would literally be on the stove, i.e., on top of it, which is not what you do with the fire.

So i is used because the stove is a container and you burn the wood inside it.

Why is vedovnen written as one word? What do ved and ovn mean separately?

Norwegian loves compound nouns, and they are normally written as one word.

ved = firewood
ovn = oven / stove
ovnen = the oven/stove (definite form: ovn
- -en)

So:

vedovn = wood stove (literally: firewood-stove)
vedovnen = the wood stove

Because it’s one concept (a type of stove), Norwegian writes it as a single compound noun: vedovnen.

Why is it vedovnen (definite form) and not en vedovn (indefinite)?

Vedovnen is the definite form: the wood stove, a specific one that speaker and listener know about (for example, the one in the house).

Vi må fyre i vedovnen = We have to light the wood stove.
→ Sounds like “our/the usual stove in this place.”

If you say:

Vi må fyre i en vedovn når det er kaldt.
= We have to light a wood stove when it’s cold.

This sounds more general or abstract, like a rule or instruction (any wood stove, not a particular one you’ve already identified).

In a real-life context, you normally talk about the one you have, so vedovnen is natural.

Why is it det er kaldt and not det er kald?

Kaldt is the neuter form of the adjective kald (“cold”), and also the form used in general / adverbial descriptions.

Patterns:

Masculine/feminine noun:
- en kald dag = a cold day
- ei kald natt = a cold night
Neuter noun:
- et kaldt rom = a cold room
Weather / general condition with det:
- Det er kaldt. = It’s cold.

When you talk about the weather or general temperature with det (“it”), you use kaldt, not kald:

Det er kaldt i dag. = It’s cold today.
Det er kaldt ute. = It’s cold outside.

So det er kaldt is the normal, correct way to say “it is cold”.

Can I change the word order to “Når det er kaldt, må vi fyre i vedovnen”?

Yes, that is perfectly correct, and very natural.

Both are fine:

Vi må fyre i vedovnen når det er kaldt.
Når det er kaldt, må vi fyre i vedovnen.

The difference is mainly emphasis:

First version: starts with what we must do.
Second version: starts with the condition (“when it’s cold”).

Grammatically, Norwegian keeps the finite verb in second position in the main clause:

Når det er kaldt, må vi fyre i vedovnen.
(The whole “Når det er kaldt” counts as position 1; må is then position 2.)

Could I say “Vi må fyre opp vedovnen” instead? What’s the difference with fyre opp?

Yes, you can say:

Vi må fyre opp vedovnen når det er kaldt.

Fyre opp is a phrasal verb meaning to start up / light something:

fyre opp vedovnen = to light/start the wood stove
fyre i vedovnen = to burn wood / have a fire going in the wood stove (can include starting it and also keeping it going)

Nuance:

fyre opp vedovnen often focuses on starting the stove from cold.
fyre i vedovnen can mean operate it generally, including adding more wood.

Both are very natural here; the original sentence doesn’t sound incomplete or wrong in any way.

What’s the difference between må, bør, and skal in a sentence like this?

They all express a kind of obligation or plan, but with different strength and nuance:

må = must / have to / need to
- Strong necessity, practical or logical.
- Vi må fyre i vedovnen = We have to light the stove (otherwise we’ll be cold).
bør = should / ought to
- Recommendation, good idea, but not absolutely necessary.
- Vi bør fyre i vedovnen = We should light the stove (it would be wise/comfortable).
skal = shall / are going to / are supposed to
- Plan, arrangement, or strong external obligation.
- Vi skal fyre i vedovnen = We are going to / are supposed to light the stove.

In your sentence, må is used because it’s about a real, practical necessity when it’s cold.

How would you say this sentence in the past and (simple) future?

Present (original):

Vi må fyre i vedovnen når det er kaldt.
= We have to light the wood stove when it’s cold.

Past (repeated situation in the past):

Vi måtte fyre i vedovnen når det var kaldt.
= We had to light the wood stove when it was cold.

Simple future idea (when it gets cold later):

Often Norwegian just uses the present for future:

Vi må fyre i vedovnen når det blir kaldt.
= We’ll have to light the wood stove when it gets cold.

More explicit “future necessity”:

Vi kommer til å måtte fyre i vedovnen når det blir kaldt.
= We are going to have to light the wood stove when it gets cold.

Why is når used and not hvis? What’s the difference?

når = when / whenever
hvis = if

In this sentence:

Når det er kaldt = when(ever) it is cold
→ This assumes that cold weather will definitely happen, and describes what you normally do in that situation.

If you said:

Hvis det er kaldt, må vi fyre i vedovnen.
= If it is cold, we have to light the wood stove.

This sounds more conditional or hypothetical: in case it is cold.

So når is used because cold weather is a regular, expected condition, not just a theoretical possibility.

Is det er kaldt only about the weather outside, or can it refer to a place like a room or house?

Det er kaldt can describe both the weather and the temperature in a place:

Weather:
- Det er kaldt i dag. = It’s cold today.
Place:
- Det er kaldt i huset. = It’s cold in the house.
- Det er kaldt på rommet. = It’s cold in the room.

In your sentence, når det er kaldt can mean:

when it’s cold (outside), or
more generally when it’s cold (in the house / in general),

and context would usually make it clear.