Translation:He assumes that his friends are here.
So this specifically means someone else's friends, rather than "Han antar att sina vänner är här?"
Well, here you have a main clause and a subordinate clause, where "han" is a subject in one and "hans vänner" is a subject in the other. "Sin" is used only for objects.
Han besöker sina vänner.
Han hoppas att hans vänner vill besöka honom.
So how would you say if he means not his own friends, but the friends of a third person? Because I had the same idea first.
How come at other times 'antar' can mean 'think' but here it means only 'assume'? I have used the word think for 'antar' with quite a few sentences, but with this one, there is always a problem. How come there is a difference with this one only?