The answer is that 'if' is omitted. And 'when' is an object to be known, meaning: if they knew the time they inquire about by saying, 'When is this promise?' It is a time that is difficult and severe, in which the fire surrounds them from behind and in front, so they are unable to push it away or prevent it from themselves, nor do they find a supporter to help them. If they were not in that state of disbelief, mockery, and haste, they would not be in such a condition. But their ignorance of it is what makes it seem light to them. It is also possible that 'knows' is left without an object, meaning: if they had knowledge and were not ignorant, they would not be hasty.

And 'when' is in the accusative case due to an implied verb, meaning: when they do not stop the fire from their faces, they know that they were upon falsehood, and this great ignorance is removed from them, meaning: they do not stop it, but it surprises them and overwhelms them. It is said of the defeated in an argument: he is bewildered. From this comes the saying: 'And the one who disbelieved was bewildered,' meaning: Ibrahim, blessings and peace be upon him, overcame the disbeliever. Al-A'mash read: 'It comes to them,' and 'it bewilders them,' in the masculine form. The pronoun refers to the promise or to the time. If you say: To what does the feminine pronoun in this reading return? I say: To the fire or to the promise, because it is in the meaning of the fire which they were promised, or in the interpretation of the number or the promised time. Or to the time, because it is in the meaning of the hour. Or to the suddenness. It was said in the first reading: The pronoun refers to the hour. Al-A'mash read: 'suddenly,' with an open 'ghain,' and 'nor are they given respite' is a reminder of His giving them respite and delaying them, and extending the time for their remembrance upon them, meaning: they are not given respite after a long delay.