"He sleeps for several days."
Translation:Han sover i flera dygn.
I answered this question lower down on the page but I'll paste my answer here too, maybe it'll be more visible.
i is the standard preposition for expressing how long something goes on (a period of time) when you'd say for in English.
Jag har varit här … 'I've been here …'
… i en vecka 'for a week'
…i ett år 'for a year'
…i en timme 'for an hour'
I feel your pain. Prepositions, conjunctions and time are WAY harder than plurals and I have hit quite a frustrating learning plateau. I've constructed a table that the tracks the fine differences in meaning for each preposition in order to detect patterns. I think I see one for på. Arnauti's comments help tremendously - I saved them as a reference.
Sorry it's taken me so long to respond - I've been in China and also trying to learn Korean without the help of Duolingo. I don't think that there is a way to upload files here and I don't think copy and paste will work. I don't have a public Facebook page, but I do have Twitter and Instagram. Let me play with those and see if I can put the document there and link you to it. Fingers crossed.
Prepositions are one of the hardest things in learning a new language. I actually find Swedish ones reasonably easy, since a lot of them (by no means all) are similar to the ones in German, my mother tongue. How hard is English (compared to German): ''in learning a language': 'by learning a language' 'beim Lernen' 'a lot of': 'many thereof' 'viele davon' 'similar to the ones in German': 'similar how the in German' 'ähnlich wie die in Deutsch' 'compared to': 'compared with' 'verglichen mit' and so on. If there are fixed expressions, learn them like vocabulary (e.g. En av dem). With others you need to develop a 'feeling', which is best done by learning some simple rules to put the right preposition and then continual practice like in duolingo, just like as a child. It needs to become 'weird' to use them differently. Some things just take their time and it will get better!
Based on an explanation from another thread, if I understand correctly, "för" the way you're trying to use it would imply something like "for the sake of several days" as of you were doing it for them, rather than "for the duration of several days" or "over a period of several days". I hope I'm explaining that correctly.
I'm pleasantly surprised that so many other people are struggling with prepositions as well. I too have felt like I've reached a learning plateau here because I just can't get past the darn prepositions and grammatical structure. If anyone has any tables or anything that they could share to help me, I would appreciate that.