وَيَقُولُونَ مَتَىٰ هَٰذَا ٱلْوَعْدُ إِن كُنتُمْ صَٰدِقِينَ · لَوْ يَعْلَمُ ٱلَّذِينَ كَفَرُوا۟ حِينَ لَا يَكُفُّونَ عَن وُجُوهِهِمُ ٱلنَّارَ وَلَا عَن ظُهُورِهِمْ وَلَا هُمْ يُنصَرُونَ · بَلْ تَأْتِيهِم بَغْتَةً فَتَبْهَتُهُمْ فَلَا يَسْتَطِيعُونَ رَدَّهَا وَلَا هُمْ يُنظَرُونَ · وَلَقَدِ ٱسْتُهْزِئَ بِرُسُلٍ مِّن قَبْلِكَ فَحَاقَ بِٱلَّذِينَ سَخِرُوا۟ مِنْهُم مَّا كَانُوا۟ بِهِۦ يَسْتَهْزِءُونَ
Wa yaqūlūna matā hādha'l-waʿdu in kuntum ṣādiqīn · Law yaʿlamu'lladhīna kafarū ḥīna lā yakuffūna ʿan wujūhihimu'n-nāra wa lā ʿan ẓuhūrihim wa lā hum yunṣarūn · Bal taʾtīhim baghtatan fa-tabhatuhum fa-lā yastaṭīʿūna raddahā wa lā hum yunẓarūn · Wa laqadi'stuhziʾa bi-rusulin min qablika fa-ḥāqa bi'lladhīna sakhirū minhum mā kānū bihi yastahziʾūn
"They say: 'When is this promise, if you are truthful?' · If only those who deny knew the hour when they will not keep the fire from their faces or from their backs, and will not be helped! · Rather, it will come upon them suddenly and stun them; they will not be able to turn it back, nor will they be given respite. · Messengers before you were mocked as well; and what they made light of closed in upon those who scoffed at them."
The grammarians call this the elision of the jawāb of law: "if only they knew…" — and the rest is not said. The elision conveys that the threat cannot be described.
I record this as an observation about the structure: the sentence is not completed, and its not being completed is a choice of narration. The same technique was treated at al-Takāthur on 102/5 (kallā law taʿlamūna ʿilma'l-yaqīn). I rely on that treatment.
عَن وُجُوهِهِمْ … وَلَا عَن ظُهُورِهِمْ — two directions: the front and the back. And the choice is worth recording: a person can protect his face; he cannot protect his back. By naming both, the verse leaves no direction to be protected.
فَتَبْهَتُهُمْ — root ب-ه-ت: to leave someone without an argument, to stun. Buhtān — slander; mabhūt — struck dumb. And in another place in the Qurʾān the same root is used of Abraham's opponent: fa-buhita'lladhī kafar (al-Baqara 2/258).
I give this as a note about the root and add this: the same root is the verbal counterpart of the scene described in the sixty-fifth verse of this sūra — Abraham's opponents falling silent and hanging their heads. The word does not occur in the sūra; the resemblance belongs to the fact.
فَحَاقَ بِٱلَّذِينَ سَخِرُوا۟ — ḥāqa — root ح-ي-ق: for a thing to come back upon its owner and close in on him. And a point of word order: what closes in is not the punishment but the very thing they mocked — mā kānū bihi yastahziʾūn.
This sentence was named at al-Shuʿarāʾ on 26/6 and it was recorded there: "when the thing mocked comes to pass, it comes no longer as news but as an event." I rely on that treatment.