What is the secret behind the difference between Surahs Yunus and An-Nahl in affirming the particle "fa'" (ف) in the apodosis (jawab ash-shart) in the verse: "If their term comes, they will not be delayed" (إذا جاء أجلهم فلا يستأخرون), while not affirming it in the verse: "So when their term comes, they will not be delayed" (فإذا جاء أجلهم لا يستأخرون)?

There is a discrepancy between Surat Al-A'raf, Surat An-Nahl, and Surat Yunus. In Al-A'raf: {And for every nation is a term; so when their term comes, they will not remain behind for an hour, nor will they precede [it]} (7:34). In Yunus: {For every nation is a term. When their term comes, they will not remain behind for an hour, nor will they precede [it]} (10:49). In An-Nahl: {But He postpones them until a specified term. So when their term comes, they will not remain behind for an hour, nor will they precede [it]} (16:61). The letter 'fa' (so/then) is attached to the conditional 'idha' (when) in Al-A'raf and An-Nahl, but not in Yunus. Al-Alusi mentioned that the 'fa' in Al-A'raf is an eloquent 'fa', and that it was omitted in Yunus because it was a response to the disbelievers' hastening of punishment. It was also said that the omission of the 'fa' is to emphasize the gravity of the matter and highlight its significance.