This was a drop-down and I got to choose between "ieder" and "iedere".
I chose "ieder" but it marked it as wrong.
Won't both work for this sentence? If no why won't ieder work?
"Ieder" is only used for het-words.
Note that the drop-down hints give all possible translations of a word, but that they won't necessarily all fit in the sentence.
Simius, are there grammar notes about all of this? ... because I can't get my head around it yet. If not - can you suggest a good link I could read over?
Yep, it is explained in the post on adjectives.