Learn Hardcore Norwegian: Læreren sier at vi må bruke linjal når vi tegner kartet. - The teacher says that we must use a ruler when we draw the map.

at introduces a subordinate clause (at vi må bruke linjal når vi tegner kartet).
It is similar to English that, and in spoken Norwegian it can be left out, especially after common verbs like si, tro, mene, vite.

What exactly does må mean, and is it the same as English must or have to?

Må is a modal verb expressing necessity or obligation. It usually corresponds to English must, have to, or sometimes need to.

In this sentence:

vi må bruke linjal ≈ we must / have to use a ruler

Key points about må:

It does not take an -r in the present tense (you never say mår).
It is followed by an infinitive verb: må bruke, må gjøre, må lese, etc.
Past tense is måtte:
– Vi måtte bruke linjal. = We had to use a ruler.

Compared to skal:

må = obligation/necessity (rules, strong advice)
skal = future, plan or weaker obligation, often from someone’s arrangement or authority:

Vi må bruke linjal. = We are required to use a ruler.
Vi skal bruke linjal. = We are going to use a ruler / We’re supposed to use a ruler.

Why is it bruke and not bruker after må?

After modal verbs in Norwegian, the main verb stays in the infinitive form, without å.

Modal verbs include: kan, vil, skal, må, bør, får.

Pattern:

subject + modal verb + infinitive (without å)

Examples:

Vi må bruke linjal. (not bruker)
Jeg kan skrive. (not skriver)
De skal lese boka. (not leser)

So:

må bruke = must use / have to use
bruker would be a normal present tense form on its own (e.g. Vi bruker linjal. = We use a ruler.), but not after a modal.

Why is there no article before linjal? Why not en linjal?

Norwegian often omits the article with tools, instruments, and means of transport when they are used in a general, practical sense.

So:

bruke linjal = use a ruler / use a straightedge (in general)

You will often see bare singular nouns (no article) in expressions like:

skrive med blyant = write with (a) pencil
spise med skje = eat with (a) spoon
høre på radio = listen to (the) radio
reise med tog = travel by train

You can say bruke en linjal, but that tends to sound a bit more like:

use one specific ruler / a single ruler, or you are emphasizing the number or identity more than the general method.

In instructions and rules, the bare form is very typical:

Du må bruke hjelm. = You must wear a helmet.
Vi må bruke linjal. = We must use a ruler.

What does når mean here, and how is it different from da?

In this sentence, når means when in a general or future-oriented sense:

... når vi tegner kartet. = ... when we draw the map.

The basic difference:

når
- Used for:
  - present and future time: Når vi tegner kartet ...
  - repeated or general situations: Når jeg er trøtt, drikker jeg kaffe.
- Often can mean when(ever).
da
- Used for:
  - one specific event in the past:
    – Da vi tegnet kartet i går, måtte vi bruke linjal.
    = When we drew the map yesterday, we had to use a ruler.

So:

Present/future or general = når
Single, specific past time = da

Why is tegner in the present tense, even though this sounds like a future action in English?

Norwegian often uses the present tense to refer to future events, when the context makes the time clear.

... når vi tegner kartet.
Literally: when we draw the map, but it functionally means when we (are going to) draw the map.

This is very common:

Jeg reiser i morgen. = I travel tomorrow / I’m leaving tomorrow.
Vi spiser når du kommer. = We’ll eat when you arrive.

English often prefers a future construction (will, going to) or a present with a future meaning. Norwegian just uses the ordinary present here, and the når-clause plus context give the future meaning.

You could mark the future more explicitly with an adverb:

... når vi skal tegne kartet senere. = when we are going to draw the map later.

Why is it kartet and not just kart? What does the -et ending do?

The ending -et makes kart definite: kartet = the map.

Norwegian usually shows definiteness by adding an ending to the noun:

et kart = a map (indefinite)
kartet = the map (definite)

Similarly:

en lærer = a teacher
læreren = the teacher

In this sentence, kartet implies a specific map, for example the map for this exercise or assignment. So:

når vi tegner kartet = when we draw the map (the particular one we know about)
når vi tegner kart would sound like when we draw maps (in general), which is not what is meant here.

How do we know that læreren means the teacher and not a teacher?

Norwegian normally marks the definite form by adding an ending to the noun.

For masculine nouns:

en lærer = a teacher
læreren = the teacher

So the -en at the end is the attached definite article (similar to the in English, but stuck on the noun).

In the sentence:

Læreren sier ... = The teacher says ...

If you wanted a teacher, you would say:

En lærer sier at ... = A teacher says that ...

Why is the word order vi må bruke linjal inside the at-clause? Is there any inversion in subordinate clauses?

Inside the at-clause (at vi må bruke linjal når vi tegner kartet), the basic word order is:

subject – verb – (other elements)

So:

vi (subject)
må (modal verb)
bruke linjal (infinitive phrase)

In main clauses, Norwegian is a V2 language: the finite verb usually appears in the second position:

Læreren sier det. (subject first, verb second)
I dag sier læreren det. (adverbial first, then verb second: sier)

But in subordinate clauses introduced by at, når, fordi, etc., you do not apply V2 in the same way. The pattern is:

[conjunction] + subject + verb + ...

So:

at vi må bruke linjal (not at må vi bruke linjal)
når vi tegner kartet (not når tegner vi kartet inside this sentence)

Only in a question or another main clause would you invert around the subject:

Når tegner vi kartet? = When are we drawing the map? (main-clause question, so inversion: tegner vi)

Can I move når vi tegner kartet to the beginning of the sentence, and what happens to the word order if I do?

Yes, you can put the når-clause at the beginning. The grammar stays correct, but the emphasis changes slightly:

Når vi tegner kartet, sier læreren at vi må bruke linjal.
= When we draw the map, the teacher says that we have to use a ruler.

Word order effects:

Når vi tegner kartet is a subordinate clause; its internal order stays subject–verb:
- vi tegner (not tegner vi here because it’s a subordinate clause).
After that clause, you start a main clause (sier læreren ...). Main clause gets V2:
- The first element is the whole når-clause.
- The finite verb sier must be in second position of the main clause:
  - Når vi tegner kartet, sier læreren at ... (correct)
  - Når vi tegner kartet, læreren sier at ... (incorrect word order in standard Norwegian)

Why is there no comma before når in ... bruke linjal når vi tegner kartet?

In modern, relatively relaxed Norwegian comma usage, short, tightly connected subordinate clauses that function almost like an adverbial phrase are often written without a comma, especially in simple sentences:

Vi må bruke linjal når vi tegner kartet.

Many texts (especially those following stricter traditional rules) would allow or even prefer a comma:

Vi må bruke linjal, når vi tegner kartet.

But in contemporary everyday writing, omitting the comma here is very common and perfectly acceptable. For a learner, it’s safe to remember:

Subordinate clauses can take a comma before them.
Short, closely linked clauses like når vi tegner kartet are often written without one, as in the example you have.

How would this sentence look in the past tense, and what changes would I have to make?

To move the whole situation into the past, you need to change the finite verbs to their past forms:

Present:

Læreren sier at vi må bruke linjal når vi tegner kartet.
= The teacher says that we must use a ruler when we draw the map.

Typical past version:

Læreren sa at vi måtte bruke linjal når vi tegnet kartet.
= The teacher said that we had to use a ruler when we drew the map.

Changes:

sier → sa (present → past of si)
må → måtte (present → past of må)
tegner → tegnet (present → past of tegne)

The nouns and the structure with at and når stay the same; only the verb forms change.