"The old man had amnesia when he died."
Translation:Den gamle mand havde hukommelsestab, da han døde.
A good rule is to use da when describing past and når when describing present or future. I have talked with 3 Danes about this and they agreed to some extent (they weren't 100% sure because they never think of grammar rules). I have seen once a sentence in a book where da was used in present tense though. Not sure if it was mistake or not.
I want to say da is for things that can be pegged to a single instance (the old man died at one discrete moment, and at the moment in question he had amnesia), while når is for repeating or continuous events (the woman would come home often, and every time she'd have tea with her mom). Not positive but that's the rule of thumb I've been going with.