لَتَرَوُنَّ ٱلْجَحِيمَ
The لَ at the head of the verse is not a simple particle of emphasis. In Arabic it is the lām al-qasam: the lām of an oath that is not spoken. There is, that is, an elided (maḥdhūf) oath, and the sentence is the answer to that oath. Its open form is roughly this: "By God, you will surely see."
The ـنَّ at the end of the verb bears the name nūn al-taʾkīd al-thaqīla (the heavy nūn of emphasis). It is the strongest instrument of emphasis that can be attached to a verb in Arabic.
For comparison: in Sūrat al-ʿAṣr, treated in the preceding section, there was also a three-layered reinforcement (oath + inna + lām). The two sūras use the same rhetorical density, and by the measure of rhetoric that density shows one thing only: the one addressed is in the position of a denier. The speech is being made in front of someone who will not believe what is said.
A note on the variant readings. In some readings the verb is read in the passive: la-turawunna — "you will surely be made to see". The direction of the meaning changes (being shown instead of seeing), the ruling stays the same. Since I cannot say with certainty which imām it belongs to, I give no name; that the reading is transmitted is recorded.
Root: ج-ح-م. The fierce blazing of fire, the flaring of embers. Jaḥmat al-nār — the fierceness of the flame.
The Qurʾān uses several names for Hell, and the emphasis of each name is different: jahannam, nār (fire), saqar, laẓā, hāwiya, saʿīr and ḥuṭama. This last will be used in al-Humaza, which comes two sūras later. Two neighbouring sūras name the same place by two different properties: al-Takāthur by the fierceness of the flame, al-Humaza by the function of breaking and crushing to pieces.
The al- at the head of the word conveys definiteness: the known, the already described. Those addressed are not hearing this word for the first time.
Through three verses the verb of knowing was repeated; in the fourth the text passes to the verb of seeing. This is the gradation set up in verse 5 being applied within the text: ʿilm al-yaqīn is turning into ʿayn al-yaqīn.
And the direction of the change is one-way. Knowledge depends on choice; seeing does not. A man can choose not to know a thing, can set his knowledge aside, can forget it, can postpone it. But he cannot choose not to see what is placed before his eyes. The sūra turns the knowledge the one addressed is fleeing into a form that cannot be fled.
From this comes the central irony of the sūra: certainty will come, but it will come at the moment when it is no longer of any use. The wish "if only you knew" in verse 5 is precisely for this — had the knowledge come in its time, something would have changed; the seeing will come in its time and will change nothing.
"There is not one of you but will come to it; this is a decree settled upon your Lord." (Maryam 19/71)
This verse carries the same all-inclusiveness as la-tarawunna'l-jaḥīm: the address admits of no exception. The exegetes differ over what the word wurūd (coming to) there means — passing over it, seeing it, or entering it. The disagreement has not been settled and I state no preference here. But the point the two verses share is beyond dispute: the seeing of Hell is a scene that comes before the division between believer and unbeliever.