While "I have some brothers" doesn't sound right here there are plenty of other examples where "some" would be perfectly acceptable as a correct answer, which is likely what's causing confusion.
I think the main problem is, at least in the UK, some and several both mean a small amount and are colloquially used more or less interchangeably depending on context.
From what I gather this isn't the case in swedish (?), but so far I've not found any good explanation as to WHY one would be used over the other. As in what distinction is used to discern when to use flera, and when to use några? Is there even any distinction in the first place? Is it just colloquial?
All I've seen is people asking why "some" isn't accepted, and others replying to the effect of "because it's wrong" with no explanation as to why. It's a tad frustrating.
I don't think I can cover all of this, but here's a start.
- flera means 'at least 2 (and preferably a few more than that)'. – Best translation: several
- några means 'a small amount or number of'. – Best translation: some or a few
- många means 'a large number of'. – Best translation: many
(Swedish definitions from the dictionary that is best at definitions, Svensk ordbok).
If we just look at the definitions Google gives us
- several = 'more than two but not many'
- a few = 'a small number of' (not defined by Google, so definition from Wiktionary instead)
- some = 'an unspecified amount or number of'
- many = 'a large number of'
I guess the reason we don't like some brothers here is that the number of brothers cannot be unspecified, it can only be unknown, which is not the same thing. Other cases of some may need to be discussed separately.
Typically, you use flera when you are making the point that you do not have just one or two. You have several. It is just short of saying många. So, flera bröder might be 3 or 4. Många bröder might be 6 or 8. Några bröder might be 3. Depends a bit on the context, intent, and emphasis of the statement. Also, flera can mean "more", as in a larger number, usually when used with än. Example "flera än" = "more than".
Perhaps, but as a native Swedish speaker I would say "Jag har flera bröder." and "Jag har fler bröder." give me two different impressions. "Jag har flera bröder." makes me think of someone having several brothers, without necessarily comparing themselves to someone else, whereas "Jag har fler bröder." makes me think the person is saying they have more brothers than another person. While "flera" can be used to mean "more (than)", I would say that use is mostly colloquial.
No, bröder does mean 'brothers'.
Plural of bröd as in 'bread' is also bröd (because bröd is an ett word). This word is rarely used, we normally speak about bröd as a mass noun.
Very few words get this vowel change in the plural. Here's a link to a list of all of them: https://www.duolingo.com/comment/5886811