Tafsir of Al-Anbiya' 21:39

His saying, the Exalted, "If those who disbelieved only knew," is an inception introduced to clarify the intensity of the terror of what they are hastening and the horror of the punishment contained therein, and that they only hasten it due to their ignorance of its nature. The preference for the present tense in the conditional, even though the meaning relates to the past, is to convey the continuity of their lack of knowledge according to the context; otherwise, the negated present tense often implies the negation of continuity. The placement of the relative pronoun (alladhina - those who) in place of a pronoun is to alert, by means of the clause that follows it, to the cause of their hastening.

His saying, the Exalted, "at the time when they will not be able to ward off the Fire from their faces, nor from their backs," is the object of "knew," according to the choice of al-Zamakhshari. It refers to the promised time which they were hastening. Its annexation to the sentence—which serves the function of an adjective that ought to be known as attributed to the described entity even by the addressee, despite the disbelievers’ denial of it—is to indicate that it is so manifest that there is no need to inform them of it; it ought, rather, to be placed in the rank of self-evident, settled truths.

The apodosis (the answer) of "if" (law) is omitted. That is: If their lack of knowledge did not persist regarding the time they hasten with their words, "When will this promise be?"—which is the time when the Fire will encompass them from every side—(they would not have hastened it). The specification of the faces and the backs, meaning the front and the rear, is because they are the most prominent sides, and because encompassing them implies the encompassing of the whole, such that they would be unable to ward it off by themselves from any of their sides, "nor will they be helped" from the side of another in warding it off, etc., for what they have done of hastening.

Al-Hufi presumed the answer to be "they would have hastened to believe," and some said "they would have known the truth of the Resurrection," and both are worthless. It was said that "law" is for wishful thinking and has no answer, which is as you see. It was also said that the answer is that "knew" is treated as an intransitive verb—meaning: if they possessed knowledge, they would not have done that.

His saying, the Exalted, "at the time when..." etc., is an inception confirming their ignorance and clarifying its continuity until that time, as if it were said: When they see what they see, they will know the reality of the situation. In al-Kashf, it is stated that it is as if it were an explanatory inception; for when knowledge was negated, it raised the question: At what time will they know? Thus, the answer was: "At a time when it will not benefit them." The apparent meaning is that "at the time when..." is the object of "knew."

Abu Hayyan said: What appears to be the case is that its object is omitted due to the indication of what precedes it—that is: If those who disbelieved knew of the coming of the promised event about which they asked and which they deemed slow in arriving. And "at the time when" is in the accusative by that omitted object. This, in my view, is not apparent.