Tafsir of Al-Kahf 18:106

"That" is an explanation of the outcome of their disbelief and all their other sins, following the exposition of their fruitless deeds. "That" functions as the predicate of an omitted subject; that is, "The matter and the affair is that." His saying, the Almighty and Majestic: “Their recompense is Hell,” is an explanatory sentence for it, and thus has no place in grammatical inflection.

It is permissible that "That" is the subject, and "their recompense" is a substitute (badal) for it—either a substitute of inclusion (ishtimal) or a substitute of the whole for the whole, if the reference is to the recompense present in the mind—and "Hell" is its predicate. The masculine form is used here, even though the predicate is feminine, because the thing referred to is the "recompense," and because the predicate, in reality, is for the substitute.

It is also possible that "That" is the subject, "their recompense" is its predicate, and "Hell" is an explanatory appositive (‘atf bayan) for the predicate, with the reference being to the Hell present in the mind.

Another possibility is that "That" is the subject, and "their recompense is Hell" is a nominal sentence functioning as its predicate, with the pronominal reference (‘a’id) omitted; the reference here is to their disbelief and their deeds, and the masculine form is used with regard to what was mentioned—that is: "That is their recompense for which [they shall have] Hell." This is countered by the argument that the omission of a pronominal reference governed by a preposition is only common in such constructions when it is governed by a preposition indicating partition (tab'id) or location (zarfiyyah), or when a preceding pronominal reference is governed by the same preposition that governs it, as in the poet's saying: "For he whom you claim [to be a savior], you are successful [through him/by him]."

Abu al-Baqa’ permitted that "That" is the subject, "their recompense" is a substitute or an explanatory appositive, and "Hell" is a substitute for "recompense" or the predicate of an omitted subject; that is, "It is Hell."

His saying, the Almighty: “for that they disbelieved,” is the predicate of "That." After mentioning these various grammatical perspectives, he stated: It is not permissible for the prepositional phrase to attach to "their recompense" due to the separation between them by "Hell." It has been said: The apparent meaning is that it does attach to it, and the separation does not cause harm in such a case; this is an explicit statement that what was mentioned is a recompense for their disbelief, which encompasses all the other abominations indicated by the Almighty’s saying—which is conjoined to "they disbelieved"—“and took My signs and My messengers in ridicule,” meaning: mocked them. For they were not content with mere disbelief in the signs and the messengers, peace be upon them, but also committed such a grave offense as well.

It is permissible that the sentence is an initiation (musta’nafah), though this is contrary to the apparent meaning. Regarding the "signs," it has been said: They are the manifest miracles performed at the hands of the messengers, peace be upon them, and the divine scriptures revealed to them, upon them be prayer and peace.