Level 1
Når or da
Use når for repeated or future time. Use da for one specific past time.
repeated or future
one past moment
Classify the time reference
Use når for repeated events and future time. Use da for one particular occasion in the past.
Both can correspond to English when, so an English translation usually supports rather than reveals the choice.
Examples
Når jeg kommer hjem, lager jeg mat.
When I come home, I make food.
habit
Da jeg kom hjem, lagde jeg mat.
When I came home, I made food.
one past occasion