"I saw him when I was in Amsterdam."
Translation:Ik heb hem gezien toen ik in Amsterdam was.
I understand that they want us to use toen in simple past, but is there a grammatical reason why wanneer is incorrect?
"Toen" refers to a one-off situation or period in the past: that time when/at the moment when:
- Toen hij belde, stond ik onder de douche - When he called, I was in the shower.
- We aten poffertjes toen we honger hadden - We ate poffertjes, when we were hungry.
"Wanneer" is a synonym for "als" and is used when the situation is something that happens usually, or at least more than once. There is no rule as to what tense this should be used in:
- Wanneer/Als je honger hebt, moet je eten - When/If you're hungry, you should eat.
- Hij snurkt wanneer/als hij slaapt - He snores when he sleeps.
Hope that makes some sense! Happy learning!