Level 1
Introduce it, then refer back
Level 1 introduces new nouns with en, ei, or et. Level 2 uses a definite ending when the listener already knows which item you mean.
en bil
a car, new in the conversation
bilen
the car, already known
New versus identifiable
Use an indefinite phrase when introducing one member of a category. Use the definite ending when the listener can identify the intended referent from prior mention or the situation.
Examples
Amir ser etter en bil.
Amir is looking for a car.
Bilen står utenfor.
The car is outside.