If the sentence is in the past tense, and the action happened once, we use da. If the action happened more than once, we use når.
| Jeg købte en bil, da jeg var i Silkeborg. | I bought a car when I was in Silkeborg. |
| Vi spiste altid kartofler, når vi besøgte min mor. | We always ate potatoes when we visited my mother. |
Når is also used in the present tense and the future tense.
| Han går i seng, når han er søvnig. | He goes to bed when he is sleepy. |
| Jeg skal købe en jakke, når jeg får penge. | I shall buy a jacket when I get some money. |
Comparison:
In English the conjunction when is used. When corresponds to når and da and is used in all tenses.