Word by Word Analysis

فَرَدَدْنَـٰهُ إِلَىٰٓ أُمِّهِۦ كَىْ تَقَرَّ عَيْنُهَا وَلَا تَحْزَنَ وَلِتَعْلَمَ أَنَّ وَعْدَ ٱللَّهِ حَقٌّ وَلَـٰكِنَّ أَكْثَرَهُمْ لَا يَعْلَمُونَ 13

So We restored him to his mother that she might be content and not grieve and that she would know that the promise of Allāh is true. But most of them [i.e., the people] do not know.