Learn Hardcore Norwegian: Barna leker innendørs når det regner.

Barn is the indefinite singular or plural form (“a child” or “children” in a general sense). When you want to say “the children,” you use the definite plural ending –a, so it becomes Barna = “the children.”

Why is the verb leker used here, and how do you form the present tense in Norwegian?

The infinitive “to play” is å leke. To form the present tense, you drop å and add –r:
å leke → leker (“play” / “is playing”).
So Barna leker means “the children play” or “the children are playing.”

What’s the difference between leke and spille? Could you use spille instead?

Use leke for children’s free play (to play with toys, run around, etc.).
Use spille for games, sports, or musical instruments (to play chess, football, piano).
Since this sentence speaks of children just playing indoors, leker is the correct choice.

Why is innendørs written as one word, and could you say inne instead?

Innendørs is an adverb meaning “indoors.” It’s a fixed word (one word, no preposition needed). You can also use the shorter inne (“inside”), which is even more common in speech:
Barna leker inne når det regner.
Both are correct; innendørs simply stresses the idea of “indoors” more formally.

Why do we say når det regner instead of something like i regn?

Når means “when” (referring to time or repeated events). To talk about weather you need a subject, so Norwegian uses the dummy pronoun det (“it”) + present tense: det regner = “it’s raining.”
Hence når det regner = “when it rains.”

How does word order work with the subordinate clause når det regner at the end?

Norwegian main clauses are SVO, so here:
Subject (Barna) – Verb (leker) – Adverbial (innendørs) – Subordinate clause (når det regner).
In subordinate clauses introduced by når, you keep the verb in second position inside that clause: når det regner (Conjunction – Subject – Verb).

Can you move når det regner to the front of the sentence? If so, what changes?

Yes. When a subordinate clause leads, it stays intact, but the main clause still follows V2 (verb-second) and its subject comes after the verb:
Når det regner, leker barna innendørs.
Notice leker (verb) comes before barna (subject) in the main clause after the comma.