Tafsir of Qaf 50:42

(The Day they hear the Cry): This is the second blast [of the Trumpet]. (And the Day) is a substitute (badal) for (the Day) mentioned in [the phrase] "the Day the Caller calls," and the governing agent (‘amil) for both of them is what is indicated by [the phrase] (That is the Day of coming forth), as previously mentioned.

It is permissible that it be an adverbial phrase for what "That" indicates, and that "the Day the Caller calls" is not governed by it but by something else, according to what has passed. It is also permissible that it be an adverbial phrase for "calls."

His saying, the Exalted: (In truth) is in the position of a state (hal) from (the Cry); meaning, they hear it while it is associated with the truth, which is the Resurrection. It is permissible that (the Truth) be in the sense of certainty, and the speech is similar to "he shouted with certainty," meaning he produced a shout with certainty, not like an echo or otherwise. It is as if it were said: "The verified Cry."

It is also permissible that the prepositional phrase be connected to "hear," in which case the meaning is "they hear [it] with certainty." It is also possible that the 'ba' (in bi-l-haqq) is for an oath, and (the Truth) is Allah, the Exalted; meaning, "they hear the Cry, I swear by Allah"—and this is as you see [it to be weak].

(That): Meaning, that Day, (is the Day of coming forth) from the graves. It is one of the names of the Day of Resurrection.

It is said: The reference is to the call, and there is a metaphorical extension in the adverbial usage, so it is made the predicate of the verbal noun; or the speech is based on the omission of an added term (mudaf), meaning: "That call is the call of coming forth," or "the time of that call is the Day of coming forth."