and avoid associating partners with Allah (shirk), which is a cause of entering Hell-fire.”
The definite article is used in “the Fire (al-nār)” because it is well-known and firmly fixed in people’s minds, having been heard about from [all] the prophets from Adam up till now. Its being governed by the relative pronoun “which (allatī)” although [such pronouns] generally refer to something previously known, is in consequence of the earlier revelation of “a Fire whose fuel is men and stones,”(66:6) before this verse, and those addressed had heard this. Thus the relative pronoun is in place.
“Whose fuel is men and stones (waqūduhā al-nās wa’l-ḥijāra):” as mentioned above, the purpose of this is to intimidate, and intimidation is made severer by threatening. Thus it threatens with the word “men” as though striking them and reinforces this with “stones” as though castigating them: “What profit or salvation do you hope for from these idols that are now the instruments of your torture?”
As for the phrase: “prepared for those who reject faith (u‘iddat li’l-kāfirīn):” Know that the position [here required] ‘prepared for you’ (u‘iddat la-kum), however, generally the Qur’an mentions summaries and general rules at the ends of verses to indicate the most powerful evidences for the statements [made in the verse in question]. Thus, what is basically [implied here] is “prepared for you if you disbelieve, since it is prepared for the disbelievers.” For this reason the noun is used in place of the pronoun.
Then “prepared” being in the perfect tense indicates that Hell exists at the present time, as was discussed above.