At-Takathur: (5) Nay, if you knew...

(Nay, if you knew with the knowledge of certainty) means: If you knew what is before you with the knowledge of that which is certain—that is, just as you know those matters which you are certain of—then the "knowledge" is in the genitive construction with the object, and "certainty" (al-yaqin) in the sense of "that which is certain" (al-mutayaqan) is an adjective for a determined noun.

Abu Hayyan permitted the possibility that the construction is of the type where the described is added to its descriptor, meaning: "the certain knowledge." The benefit of this description is manifest, based on the fact that "knowledge" can be applied to that which is not certain.

The response to "if" (law) is omitted for the sake of intensification; that is: "If you knew in such a way, you would have done things that cannot be described or fathomed," or "that would have occupied you away from competition (in worldly gains) and other things," or something similar.