"These lions are sad, because they are in a zoo."
Translation:Te lwy są smutne, bo są w zoo.
I went with "bo", but would like to know as well. I thought they were completely interchangeable, just more or less appropriate for certain uses?
If you write "bo" on the grammar test in school teacher would mark it as incorrect, because it is a colloquialism. But in everyday use they are completely interchangeable and no one notices the difference.
oni - personal-masculine plural - only used with human males
one - non-personal-masculine plural - used with everything else.
Lions are not people => One