Tafsir of Al-`Ankabut 29:55

(The Day the punishment will cover them) is an adverbial phrase for an implied [verb] that has been omitted, in order to signal the extremity of its abundance and horror. It is as if it were said: "On the day the punishment—which was alluded to by the encompassing of Hell upon them—comes to them and overwhelms them, there shall be such states and terrors that speech cannot suffice to describe."

It is also said: It is an adverbial phrase for "encompassing" (muḥīṭah), in the sense of: "And indeed, Hell shall encompass the disbelievers on the day the punishment covers them from above them and from beneath their feet." That is, from all their directions; so what is mentioned is for the sake of generalization, as in [the verse] "in the mornings and the afternoons." It is said: Mentioning the "feet" is to indicate that they shall neither settle nor sit, and that is the severest of punishment.

(And He will say), that is, Allah—Mighty and Majestic is He. It is also said: The angel appointed over them.

Ibn Kathīr, Ibn ‘Āmir, and the Basrans read it as (And We will say) with a nūn denoting majesty, and it is apparent that the Speaker is Allah the Exalted.

Abū al-Barhasm read it as (And she will say) with a tā’, implying that the speaker is Hell. The speech is attributed to it here just as it is attributed in His saying—Exalted be He—: "(And she will say: 'Are there any more?')."

Ibn Mas‘ūd and Ibn Abī ‘Ablah read it as (And it will be said) in the passive voice: "Taste what you used to do"—that is, the recompense for what you used to do continuously in the world, [namely] the evil deeds, among which is the hastening of the punishment.