"He went to a restaurant to eat dinner with his guests."
Translation:Li iris al restoracio por vespermanĝi kun siaj gastoj.
Why is it "siaj gastoj" and not "liaj gastoj" ? I'm affraid I can get it : it is a singular masculine, right ?
siaj means that the guests belong to the (third-person) subject.
If you had used liaj, the guests would have belonged to some other male.
So it is some kind of reflexive rule ? When the possessive refers to the subject, it is always sia ?
Only if the subject is third person (he, she, it, they).
So this is more like Swedish than like Russian -- we don't say Mi legas sian libron for "I read my (own) book".