Learn Hardcore Norwegian: Først når alle er på oppmøtestedet, forklarer vi sitteplanen og kodeordet. - Only when everyone is at the meeting place do we explain the seating plan and the code word.

Breakdown of Først når alle er på oppmøtestedet, forklarer vi sitteplanen og kodeordet.

What does Først når mean here? It looks like first when.

In this sentence, først når does not mean a literal time sequence like first, when.... It means something closer to:

So Først når alle er på oppmøtestedet, forklarer vi ... means:

This is a very common Norwegian pattern.

Why is it forklarer vi and not vi forklarer?

Because the sentence starts with a subordinate clause:

After that comes the main clause, and Norwegian follows the V2 rule in main clauses: the finite verb must come in the second position.

So the structure is:

If you wrote the main clause by itself, it would be:

But when something else is placed first, the verb moves before the subject:

This inversion is very typical in Norwegian.