Nay but bal is for effecting a transition from one object to another which in this case is to inform of his state and theirs you marvel ‘ajibta addressing the Prophet s that is you marvel that they deny you while they engage in ridicule at your marvelling
- 37:1212
بَلْ عَجِبْتَ وَيَسْخَرُونَ
Nay, but whereas thou dost marvel, they [only] scoff;
- common.revelation-Meccan
- common.hizb : 178