from what I understand "opened" is used only as a verb or as a participle that is accompanied by a verb e.g. "is opened by sb", but when a participle is on its own it's always "open", like the adjective, lookie here: http://dictionary.cambridge.org/grammar/british-grammar/open-or-opened the rules for participles are similar
There is a slight difference; 'an opened door' is a door which somebody opened at some point, while 'an open door' is simply an open door. This difference exist in Norwegian, and it's better to translate to the closest word to translate the meaning. 'åpnet', much like 'opened', is uncommon in Norwegian, and 'åpen'(open) is preferred in most cases.
yeah, I understand the difference, I'm just not convinced that it is correct to write that in English. Maybe it is, it looked very weird to me so I did a little research and I found several people saying pretty much what I wrote above. So I thought I'd write something here, but I'm no expert, just trying to help :)
I know, I realised that yesterday evening when I remembered "I got an opened letter" from the Swedish course. I don't know why I had a problem with "door", maybe because you don't hear the "-ed" when it's not stressed, it kind of fades into the "door" and disappears. Anyway, sorry guys, false alarm. The saddest part is that so many people out there seemed to agree with my stupid thinking :/
Oh, and Luke, thanks for this AWESOME course! I'm finishing my tree today and I'm starting to work my way up to 25 :)