Breakdown of Når det lyner, går vi indenfor.
det
it (neuter)
vi
we
gå
to walk / to go / to go out, to fail (of power)
når
when
Questions & Answers about Når det lyner, går vi indenfor.
Why is når used here instead of da for "when"?
In Danish, når is used for things happening in the present or future, or for repeated events in the past (meaning "whenever"). Because this sentence describes a general rule, we use når. You only use da for a single, specific event in the past.
Why does the second part say går vi instead of vi går?
This is the V2 (verb-second) rule in action. The entire first clause (Når det lyner) counts as the first position in the sentence. Therefore, the verb of the main clause (går) must come immediately after it in the second position, followed by the subject (vi).
How does the word lyner work here?
While English often uses a noun phrase like "there is lightning," Danish uses an active verb, at lyne. Just like with rain (det regner) or snow (det sner), you use the dummy subject det followed by the active weather verb.
Could I use inde instead of indenfor here?
No, because the sentence describes movement from one place to another. Indenfor implies direction, showing that you are moving to the inside. You use inde when there is no movement and you are already located inside (like vi er inde).