"One animal, several animals"
Translation:Ett djur, flera djur
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"Flera" can be used in different ways and sometimes it works to replace it with "fler". When it means many like above, it should always be "flera" though.
For comparison, both work fine:
Jag har fler böcker än du (Jag har flera böcker än du) = I have more books than you.
I prefer "fler" in this sentence.
In Swedish, "ett" words are usually the same for singular and plural (ett barn, flera barn / ett djur, flera djur). It's the same for a number of English words like sheep, fish, moose etc. They are the same for singular and plural. I ate a fish. I caught 3 fish. I saw 6 sheep today.