I don't know if I got it right in the first place, but one of my notes says
- om + definite form - means 'during the' [winter, summer, ...]
- i + indefinite form - 'I sommer skal vi på ferie'. Meaning is the same as above I guess, because 'I flere hundre millioner år' - 'For several hundreds of millions of years'. Here 'for' means 'during'
- på [tirsdager] - usually used for repeatable actions
'I flere hundre millioner år' is a sentence from Duo Norwegian course. Within the course it is translated with using 'for' - 'For several hundreds of millions of years'. Since I'm not a native English speaker and can't really appreciate all the nuances of using 'for' instead of 'during' in this particular context, yet the meaning of it is still 'during' to me. At least this is how I would translate it to Ukrainian or Russian.
What I was trying to say is that you can use both 'om' and 'i' when you want to say that something is/will be/was happening during a certain period of time. The difference is that you use 'om' with definite nouns and you use 'i' with indefinites.
Hope this make sense now. Sorry for the confusion
Okay, I get it :) For means kinda during the whole like all the time during said period (like in for 100 years and during the whole 100 years though I know it's not correct English but I'm just explaining) so you're not so off :) I'm Polish so the grammar is similar and now I think I'd translate it the same into Polish but I learned not to learn languages by translating them into your native. That doesn't really work. That's a tip from me ;) I love Russian btw :)
unfortunately Duo doesn't allow the latin alphabet spelling in the answers (tho I know you mean comments now). It's be cool if it would, it's be easier to write not having to use mouse to click on those few letters. If Russian course allows latin alphabet spelling (which is ridiculous to me because it's never used outside some courses) courses that have just a few different letters should be allowed too to use simplified version. Just my opinion though.