1) Ἰδὼν δὲ τοὺς ὄχλους ἀνέβη εἰς τὸ ὄρος, καὶ καθίσαντος αὐτοῦ προσῆλθαν αὐτῷ οἱ μαθηταὶ αὐτοῦ· (Matt 5:1)
Translation: Now, after beholding the crowds, he went up into the mountain, and after he sat down, his disciples came to him. (The phrase καθίσαντος αὐτοῦ is a genitive absolute participial phrase, so it describes the circumstance under which the main clause “his disciples came to him” occurred; in Greek it is syntactically unconnected from the main clause.)
Parsing: αὐτοῦ – Genitive, singular, masculine, αὐτός
αὐτῷ – Dative, singular, masculine, αὐτός
2) καὶ στήσω αὐτοῖς διαθήκην αἰώνιον τοῦ εἶναί με αὐτοῖς εἰς θεὸν καὶ αὐτοὶ ἔσονταί μοι εἰς λαόν καὶ οὐ κινήσω ἔτι τὸν λαόν μου Ισραηλ ἀπὸ τῆς γῆς ἧς ἔδωκα αὐτοῖς (Bar 2:35)
Translation: And I will establish with them an eternal covenant in order that I might be God to them and they might be to me a people and I will not remove again my people Israel from the land that I gave to them.
Parsing: αὐτοῖς – Dative, plural, masculine, αὐτός
αὐτοὶ – Nominative, plural, masculine, αὐτός