Tafsir of Al-Kahf 18:3

Abiding therein (i.e., remaining in the reward) forever (i.e., without an end to the duration of their stay).

The word mākithīn (abiding) is in the accusative case (naṣb) as a circumstantial qualifier (ḥāl) referring to the pronoun in the prepositional phrase lahum (for them). Both adverbial phrases relate to it. The warning is presented before the glad tidings to demonstrate the utmost concern for restraining the disbelievers from that which they are upon, while observing the principle of "clearing" (takhliyah) before "adorning" (taḥliyah), and the reiteration of the warning in His, the Exalted’s, saying...