Why is it toen not wanneer here?
Both words can be translated to "when" in English, but in Dutch they serve different functions. "Toen" is used to refer to past events, "wanneer" for questions.
Great -I understand now.
Could the same sentence be written as "Toen ik daar was, regende het"? Or is it "Toen ik daar was, het regende"? I can't remember how the sub-clause affects the sentence in this case...
Yes, it could be written as "Toen ik daar was, regende het".
Is it wrong to say "Het regende als ik daar was"?
I think you need to use "toen" instead of "als" since this is a past event.