Så lenge barna sitter stille i klasserommet, kan læreren forklare planen.

Here så lenge means as long as. It introduces a condition: the teacher can explain the plan provided that the children stay quiet.

Depending on context, så lenge can also mean something closer to while, but in this sentence the conditional sense is the most natural one.

Because Så lenge barna sitter stille i klasserommet is a subordinate clause placed before the main clause.

A comma is normally used in Norwegian between a fronted subordinate clause and the main clause:

• Så lenge barna sitter stille i klasserommet, kan læreren forklare planen.

This is because of normal Norwegian V2 word order in main clauses.

The first element in the sentence is the whole subordinate clause:

• Så lenge barna sitter stille i klasserommet

After that, the finite verb of the main clause must come next:

• kan

Then comes the subject:

• læreren

So:

• Så lenge ..., kan læreren ...

If you started directly with the subject, you would get:

• Læreren kan forklare planen ...

Both are correct, but the word order changes when something else comes first.

Barna is the definite plural of barn.

Useful forms:

• et barn = a child
• barn = children
• barna = the children

So barna means the children.

Sitte stille is a very common expression meaning sit still or sit quietly.

It is more specific than være stille:

• være stille = be quiet
• sitte stille = sit still / remain seated quietly

In a classroom context, sitter stille suggests that the children are not moving around and are behaving calmly.

In sitter stille, stille is best understood as a word describing the state of the subject after the verb sitte. Norwegian often uses adjective forms after verbs like være, bli, stå, ligge, and sitte where English might use an adverb or a different structure.

So English may say:

• sit still
• sit quietly

But Norwegian naturally says:

• sitte stille

Because kan is a modal verb, and after modal verbs Norwegian uses the infinitive without å.

So you get:

• kan forklare
• vil forklare
• skal forklare
• må forklare

Not:

• kan å forklare

This is very similar to English:

• can explain not
• can to explain

Here kan most naturally means can / is able to / has the opportunity to.

The idea is that if the children are sitting quietly, then the teacher is able to explain the plan properly.

It does not sound mainly like formal permission here. It is more about practical possibility.

I klasserommet is a prepositional phrase telling you where the children are sitting.

• i = in
• klasserommet = the classroom

So it means in the classroom.

It belongs most naturally with sitter:

• barna sitter stille i klasserommet = the children are sitting still in the classroom

Norwegian often uses the definite form when the speaker assumes the listener knows which person or thing is meant.

Here that gives:

• læreren = the teacher
• klasserommet = the classroom
• planen = the plan

The definite ending is attached to the noun:

• lærer → læreren
• klasserom → klasserommet
• plan → planen

This is one of the most basic patterns in Norwegian noun grammar.

It is in the present tense:

• sitter
• kan

But in Norwegian, the present tense is often used for general situations, habits, and near-future meaning too. So the sentence can describe:

• a general rule
• something that usually happens
• a situation happening now

Yes. You could also say:

• Læreren kan forklare planen så lenge barna sitter stille i klasserommet.

That version starts with the main clause, so the normal subject-verb order appears:

• Læreren kan ...

The meaning is basically the same. The original version simply puts more focus on the condition first.

Grammatically, planen simply means the plan. What specific plan it is depends on context.

In a school setting, it could mean:

• the lesson plan
• the plan for the day
• instructions for an activity

Norwegian often leaves that kind of detail to context, just like English does.