Tafsir of Az-Zalzala 99:4

"On that day" is a substitute for "when" (idha). As for His saying, "it will report its news," it refers to the earth. The possibility that the subject of the verb is the person addressed—as al-Tabrisi claimed—has no basis. The operator for both is the same. It has been said that the operator is implicit, indicated by the content of the subsequent sentences; the estimation being: "They will be gathered when [the earth] is shaken."

"On that day" is linked to "will report," while "when" is used merely to denote the timeframe. It has also been said that it is in the accusative case as the object of a deleted verb; that is, "Remember that time," thus it is neither adverbial nor conditional. It is also permitted that it is conditional, governed by an implied answer, meaning: "There will be [events] whose essence cannot be comprehended," or something similar.

The intended meaning is: On the day when it is shaken by its shaking, and it brings forth its burdens, and man says, "What is the matter with it?" it will report to the creation the news it holds. This occurs by Allah, the Exalted, creating life and perception within it, whereby it speaks literally and testifies to the obedience or disobedience that was performed upon it. This is the view of Ibn Mas’ud, al-Thawri, and others.

Supporting this is the hasan (good), sahih (authentic), and gharib (singular) hadith. Imam Ahmad and al-Tirmidhi narrated from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) recited this verse, "On that day, it will report its news." Then he said: "Do you know what its news is?" They said: "Allah and His Messenger know best." He said: "Its news is that it will testify against every male and female servant regarding what they did upon its surface. It will say: 'He did such-and-such on such-and-such a day.' This is its news."