"And let not those who disbelieve think that We only grant them respite for the good of their own selves." This is a conjunction attached to His (the Exalted's) saying: "And let it not grieve you." The verb is attributed to the relative pronoun (al-mawsul), and the particle an (that) along with its operative force acts as a substitute for its two objects according to Sibawayh, because the intended meaning—the attachment of the acts of the heart to a relation between a subject and a predicate—is achieved. According to al-Akhfash, the second object is omitted.
Ma (in annama) is either a masdariyyah (infinitive) particle or a relative pronoun. In both cases, it should ideally be written separately, but it is written as one word (annama) in the Imam (the Uthmani codex), and following the Imam is obligatory. Perhaps the reason for this is to match what follows it, or because it is the more common usage in such structures. Khayrun (good) is the predicate; it is also read as khayran in the accusative case, assuming li-anfusihim (for their own selves) is the predicate. Lahum (for them) is a clarification or a state (hal) from khayrun.
Imla' (respite) literally means prolonging a duration. Al-mala' is a long period, hence al-malawan (the two prolongers) refers to night and day due to their long, recurring succession. As for the imla' of a book (dictation), it is named as such because of the duration of time spent pausing at every word. It is also said that imla' means to leave one to their own affairs; it is said that one "grants respite" (amla) to his horse by loosening its tether so it may graze as it wishes.
The gist of the construction is: "Let not the disbelievers think that Our granting them respite, or that which We grant them as respite, is 'good for their own selves'," or "Let not the disbelievers think that the goodness of Our granting them respite is fixed or real." The implication is to forbid them from rejoicing in the outward appearance of Allah granting them long lives, or leaving them to their own devices, based on their assumption that it is for their own good. It serves to frustrate them by clarifying that such things are pure joy in appearance but absolute harm in reality.
Hamza read it as "And let you not think" (la tahsabanna) in the second person. The address is either to the Messenger (peace and blessings of Allah be upon him), which is most appropriate for the context of consolation—though the intent is to deride them for having held such an assumption—or it is addressed to anyone capable of such thinking, with the intention of spreading awareness of the atrocity of their state. The relative pronoun is the object, and "that We grant them respite" etc., acts as a substitute for it (badal ishtimal). Since the substitute is the primary intended object, and here it fills the place of two objects, it is permissible to limit it to one object; otherwise, the omission would not be correct according to the sound view.
It is possible that annama nuhli is a second object, but since it is interpreted as an infinitive (masdar), it cannot be treated as a reference to entities (dhawat), so it is necessary to provide an estimation. In the first case: "Do not think the condition and state of those who disbelieve [is such]." In the second: "Do not consider the disbelievers to be of those who possess [the attribute of] 'that which We grant them as respite'..."
The good was qualified by the words "for their own selves" because respite is a good thing for the believers, as it contains immense benefits. Those who consider "good" (khayran) here to be a superlative (af'al tafdil) and consider the object of comparison to be martyrdom in the cause of Allah, base this comparison on the disbelievers' own false assumption and argumentation. This verse was revealed regarding the polytheists of Mecca, which is what is narrated from Muqatil, or regarding [the tribes of] Qurayza and al-Nadir, which is narrated from 'Ata'.
"We only grant them respite that they may increase in sin" is an interruption providing the cause for the previous ruling. Those who hold that good and evil are by the will of the Exalted allow for such causality, whether it is an objective (gharad) or simply an intended consequence along with the action, resembling a cause for those who do not allow the acts of the Almighty to be motivated by objectives. As for the Mu'tazila, although they affirm causality, they hold that the ugly/evil is not intended by the Almighty, nor is it a sought-after objective. Hence, they considered the increase in sin here to be an incentive—similar to saying "I sat out of the war out of cowardice"—rather than an objective intended to be achieved. Since the increase is not prior to the granting of respite here, and an incentive must precede the act, they treated it as a metaphor based on the fact that its precedence in the eternal knowledge of Allah—from which the known outcome cannot deviate—is likened to the precedence of an incentive in reality. The strained nature of this interpretation is obvious, which is why it was said that it is easier to hold that the lam (in li-yazdadu) indicates the consequence (al-'aqibah).
This has been challenged by saying that even if it is less labored, it is incorrect, because this sentence is the cause for what preceded it. If the granting of respite were for a valid purpose that results in this corrupt and ugly outcome, it would not be correct, and this would not serve as a proper justification for forbidding them from thinking that respite is good for them. Contemplate this. This was stated by some of the verifiers.
Yahya ibn Wathab read annama (in this clause) with a fatha and the first one with a kasra, using the third-person prefix for yahsaban (thinking), with "those who disbelieve" as the subject. "That We grant them respite that they may increase in sin" stands in place of the two objects of thinking. The meaning is: "And let not those who disbelieve think that Our granting them respite is for the increase of sin, but rather for repentance, entering into faith, and making amends for what has passed." And "Verily, We grant them respite [which] is good for themselves" is an interpolation between the verb and its object. Its meaning is that Our respite is good for them if they wake up and repent. The difference between the two readings is that in this reading, the respite is for the sake of repentance, and respite for the sake of increasing in sin is negated. In the other reading, it is affirmed, and the other is negated by implication. There is no contradiction between them, as according to the Ahl al-Sunnah, it is permissible to intend either, and the intended outcome does not necessarily fail to materialize from the intention, as it is subject to conditions, as you have known.
Some claimed that the sentence "Verily, We grant them respite is good for themselves" is a state (hal), meaning: "Let them not think this while in this state," and that this state is contradictory to it. There is nothing to this claim.
"And for them is a humiliating punishment" is a new sentence explaining their state in the Hereafter following the explanation of their state in the world, or it is a state related to the pronoun in "they," meaning: "so that they may increase in sin—there is prepared for them a humiliating punishment." This is definite in the latter reading, as held by more than one verifier, so that the content of that becomes included in the domain of the prohibition against thinking, as if it were said: "[Let them not think this] so that they may increase in sin and so that they may have a humiliating punishment." Some made it a conjunction to the sentence "that they may increase," such that "humiliating punishment" is the subject of the prepositional phrase (la-hum) with an implied existence, which is a position of great weakness. Yes, it was said that it is permissible for it to be an interruption, and there is an aspect of merit in that generally. Furthermore, their punishment was described as "humiliating" because, as the Shaykh al-Islam said, since the granting of respite entails enjoying the good things of this world and its adornments—which invites haughtiness and arrogance—it was described as such so that their recompense would be a matching requital. It is also possible to say that this is an indication to refute that which could be a source for their assumption, namely that they are dear to Him (the Almighty), following the indication to refute it by another type.