How can I teach the difference between one's own things and others' things?

The concept of ownership (mine vs others') grows gradually. First, label all belongings with a name or mark (a favorite symbol or color) so "this is ○○'s mark" is clear. Practice concretely, in play, the steps "ask 'can I borrow?' then get a yes." If they take someone's thing, rather than scolding, return it together on the spot—"this is ○○'s; let's give it back"—and show the right way. It sinks in through repetition.