Tafsir of Luqman 31:5

This is an initiation [of a new sentence]. If all commendable deeds are intended by it—and the specification of those mentioned [previously] is for their superior significance—then the relative pronoun [in the previous verse] is the subject (mubtada'), and the sentence "Those are upon guidance..." etc., is its predicate. The statement is an initiation by way of mentioning the attribute that necessitates worthiness.

It is said that the relative pronoun, under both estimations, is an adjective; however, under the first estimation, it is clarifying (kashifa), and under the second, it is an attributive adjective for the description, not for the described. The construction of "they are certain" (yuqinun) upon "they" (hum) is for emphasis, and the pronoun is repeated for corroboration and to dispel the illusion that "of the Hereafter" is the predicate, and to compensate for the separation between the subject and its predicate. The separator was not delayed for the sake of the verse ending (fasilah).

Some prominent exegetes mentioned regarding the Almighty’s saying at the beginning of Surat Al-Baqarah: "And of the Hereafter they are certain," that the construction of "certain" (yuqinun) upon "they" (hum) indicates that their opposites are not in the shadow or shade of certainty, and that the precedence of "of the Hereafter" indicates that what their opposites are upon is not of the Hereafter in any way. This is to convey the exclusivity [resulting from] the precedence of the semantic subject and the precedence of the prepositional phrase over that to which it relates. Consider, then, whether something similar to that is possible here. The reasoning behind the choice of the demonstrative pronoun and the reason for its repetition has already been addressed at the beginning of Surat Al-Baqarah. There is further discussion regarding the verse that is not hidden from one who refers to what they have mentioned regarding similar verses there. Reflect, then refer and reflect.