Tafsir of An-Nazi'at 79:35

(The day when man shall remember what he strove for)

It is a substitution (badal) for the whole or a part of "it" ('idha ja'at), as has been said. It is also said to be a substitution for "the greatest calamity" (al-tamma al-kubra), and thus it would be in the nominative case; it is indeclinable (mabni) with a fat-hah because it is appended to a verb, according to the opinion of the Kufans. In this view, the "calamity" is in reality the remembering and the manifestation [of deeds], for the goodness of one’s deeds overcomes every pleasure, and the opposite of that [bad deeds] overcomes every hardship. Likewise, the manifestation of Hell, when one is afflicted by it, overcomes every hardship, and when one is saved from it, it overcomes every pleasure. However, the forced nature of this interpretation is not hidden.

It is also said that it is an adverbial modifier (zarf) for "came" (ja'at). Al-Tabarsi holds this view and considers it most likely that it is in the accusative case, governed by an implied "I mean" (a'ni), as an explanation for "the greatest calamity."

"What" (ma) is a relative pronoun, and "strove for" (sa'a) means "worked." The referent pronoun ('a'id) is implied—that is, "for him" (lahu). The meaning is: the day everyone remembers what he has done, whether good or evil, by seeing it recorded in his scroll. He had forgotten it due to excessive heedlessness, the length of time, the intensity of what he encountered, or the sheer volume of his deeds, which exceeds the memory's ability to retain, according to the Almighty’s saying: "Allah has recorded it, while they forgot it." It is also possible that his remembrance occurs in another way. It is also permitted that "what" (ma) be an infinitive particle (masdariyyah), meaning: "he remembers his striving therein."