Brødristeren er varm, så jeg legger brødet i matboksen med en gang.

Norwegian normally writes compound nouns as one word. So brød + rister becomes brødrister.

Then the definite ending -en is added:

• en brødrister = a toaster
• brødristeren = the toaster

This is very common in Norwegian. English often uses two words where Norwegian uses one compound.

Those endings mark the definite singular, and they depend on the noun’s gender.

• en brødrister → brødristeren
• en matboks → matboksen
• et brød → brødet

So brødrister and matboks are common-gender nouns, while brød is a neuter noun.

Because the sentence is talking about a specific bread item that is already understood in the situation.

• brød = bread / some bread
• brødet = the bread

Norwegian often attaches the definite article to the end of the noun instead of using a separate word like the.

Because brødristeren is singular and common gender.

Adjectives after er agree with the noun:

• en brødrister er varm
• et hus er varmt
• husene er varme

So varm is the correct form here. Even though brødristeren is definite, the predicate adjective is still varm, not varme.

Because the sentence contains two full clauses:

• Brødristeren er varm
• jeg legger brødet i matboksen med en gang

In standard Norwegian, you normally use a comma between coordinated main clauses like this.

Here så means so, and it links two main clauses. After this kind of så, Norwegian keeps normal clause order:

• så jeg legger

That is subject + verb.

If you used a fronted adverb like derfor, you would get inversion:

• Derfor legger jeg brødet i matboksen.

So så jeg legger is the expected pattern in this sentence.

Legger is the present tense of å legge, which often means to lay or to put.

Here it means put:

• jeg legger brødet i matboksen = I put the bread in the lunch box

A very important contrast is:

• å legge = to place something
• å ligge = to be lying / to be located

Compare:

• Jeg legger brødet i matboksen = I put the bread in the lunch box
• Brødet ligger i matboksen = The bread is in the lunch box

Norwegian often uses the present tense for actions happening now, for habits, and also for very immediate future actions.

So jeg legger here can sound like:

• I put
• I’m putting
• I’ll put

The context med en gang makes it feel immediate: the speaker is doing it right away.

In Norwegian, i can often mean both in and into when the verb already makes the movement clear.

Since legger shows that something is being placed somewhere, i matboksen naturally means into the lunch box.

You can also say inn i matboksen, but that puts extra emphasis on the movement inward.

It is not på matboksen, because that would mean on the lunch box, not inside it.

It is a fixed expression meaning right away, immediately, or at once.

So:

• med en gang = immediately

You should learn it as a whole phrase.

Yes. Matboks is a very normal Norwegian word for a lunch box or food container.

It is another compound noun:

• mat = food
• boks = box/container
• matboksen = the lunch box

There is no exact English equivalent, but a useful approximation is the vowel in some pronunciations of bird or burn, with rounded lips.

So the important point is:

• the tongue is in a fairly central/front position
• the lips are rounded more than in English

That ø sound appears in both brød and brødristeren.