﴿ بَلْ ظَنَنتُمْ أَن لَّن يَنقَلِبَ ٱلرَّسُولُ وَٱلْمُؤْمِنُونَ إِلَىٰٓ أَهْلِيهِمْ أَبَدًۭا وَزُيِّنَ ذَٰلِكَ فِى قُلُوبِكُمْ وَظَنَنتُمْ ظَنَّ ٱلسَّوْءِ وَكُنتُمْ قَوْمًۢا بُورًۭا ﴾

“Nay, you thought that the Apostle and the believers would never return to their kith and kin: and this seemed goodly to your hearts. And you entertained [such] evil thoughts because you have always been people devoid of all good!””

Nay bal in both instances marks a transition from one subject to another but you thought that the Messenger and the believers would never return to their families and that thought was adorned in your hearts that is to say you thought that they would be annihilated by being slain and would not return and you thought evil thoughts that just mentioned and other things and you were a ruined lot’ būr is the plural of bā’ir in other words a lot to be destroyed according to God for having entertained such evil thoughts.