Tafsir of An-Nazi'at 79:8

"Hearts on that Day shall be trembling," meaning on the day when the earth and the mountains quake, the hearts shall tremble—that is, they shall be agitated. It is said: "The heart wajafa (trembles) with wajifan," meaning it is agitated due to the intensity of fear; likewise, wajaba (beats) with wajiban (beating).

It has been narrated from Ibn Abbas that wajifah means "frightened" in the dialect of Hamdan. According to al-Suddi, it means "shaken." The word yawma'idhin (on that Day) is not to be taken as the object of wajifah in the accusative case, because it is the adverbial time (zaraf) for it. Founding a new meaning is better than mere emphasis, so it should not be interpreted as such; moreover, the deletion of the genitive construct (mudaf) and the substitution of the tanwin (vowel sign) are things that also reject that interpretation.

The word qulub (hearts) is in the nominative case as an initial subject (mubtada'), and yawma'idhin is connected to wajifah, which serves as the predicate, according to one opinion, and this is the most apparent view—as in His saying (Exalted is He): "Some faces on that Day shall be radiant, looking at their Lord," and "Some faces on that Day shall be gloomy."

The initiation of the sentence with an indefinite noun is permissible here because its indefiniteness serves the purpose of categorization (tanwi'), which takes the place of a qualifying description. Indeed, the categorization in the parallel verses is more apparent due to the mention of the opposite, unlike in the case we are discussing; however, there is no difference once the context drives the meaning toward it. If you wish, you may consider it for the purpose of multiplication (takthir), just as it was considered in the expression "He bought a fanged viper." It has also been said that wajifah is an adjective for qulub, which justifies beginning the sentence with it.