Questions & Answers about दोस्तों में कलह है।
Because में is a postposition, and nouns before postpositions usually take the oblique form.
- dictionary form: दोस्त
- plural direct: दोस्त
- plural oblique: दोस्तों
So दोस्तों में means among the friends or among friends.
It often does mean in, but with a plural noun referring to a group, it can also mean among.
So दोस्तों में कलह है literally means something like:
Among the friends, there is discord.
That is why में works naturally here.
Because the main thing being said to exist is कलह—discord/strife/quarrel—and that noun is singular here.
The phrase दोस्तों में is just the setting: among the friends.
So the structure is basically:
Among the friends + discord + is
Since कलह is singular, Hindi uses है, not हैं.