English has one when. German splits it in two:

• als — one event, or one stretch of time, in the past: Als ich nach Berlin kam, war ich nervös. Als ich klein war, wohnte ich in Boston.
• wenn — whenever, every time: Wenn es dunkel wurde, rief meine Mutter. — Whenever it got dark, my mother called. It often comes with immer: Immer wenn ich Oma besuchte, … — Every time I visited Grandma, …
• wenn also covers the present and the future: Wenn du Zeit hast, komm vorbei! — When you've got time, come round!

A quick test for the past: could you say every time? Then it's wenn. Did it happen once — or is it one period of your life? Then it's als.

Don't confuse either of them with wann, the question word: Wann kommst du? — When are you coming?