This is a mistake a lot of natives struggle with. "Da" is past, something that has already happened. "Når" is future, something that has yet to happen, or somethings that happen multiple times
"Da jeg kom hjem drakk jeg vann"- when I came home I drank water "Når jeg kommer hjem drikker jeg (alltid) vann"-when I come home I (always) drink water "Når jeg kommer hjem skal jeg drikke vann"-when I come home i am going to drink water.
A common rule that you learn in school is: Den gang da, hver gang når