فَلَمَّا ذَهَبُوا۟ بِهِۦ وَأَجْمَعُوٓا۟ أَن يَجْعَلُوهُ فِى غَيَٰبَتِ ٱلْجُبِّ وَأَوْحَيْنَآ إِلَيْهِ لَتُنَبِّئَنَّهُم بِأَمْرِهِمْ هَٰذَا وَهُمْ لَا يَشْعُرُونَ · وَجَآءُوٓ أَبَاهُمْ عِشَآءً يَبْكُونَ
"When they took him away and agreed to drop him into the bottom of the well, We revealed to him: 'You will surely inform them of this affair of theirs when they are not aware.' And they came to their father in the evening, weeping."
There is a grammatical event in the verse, and the lexicographers dwell on it: the conditional clause opening with fa-lammā (when…) is given no apodosis.
In Arabic this is called ḥadhf jawāb al-lammā (the omission of the answer to lammā), and the lexicographers record that it is done to leave the horror of the scene to the reader. That is, the part "when they dropped him into the well… this and this happened" is not in the text.
I record this as an observation about the structure, and I add that the technique is used once more in the sūra: the same gap is there in the scene of the drawing out of the well (19-20) — how long Joseph stayed in the well, what he felt, what he said, is never told.
And the second structural note of the story is here: to the child dropped into the well a piece of news is given, and the content of that news belongs to the future: la-tunabbiʾannahum bi-amrihim hādhā — "you will surely inform them of this affair of theirs."
I record this as my own reading, and its ground is verses 89-90: the promise is fulfilled to the letter at the end of the sūra — hal ʿalimtum mā faʿaltum bi-Yūsufa wa-akhīhi idh antum jāhilūn (89). That is, the sentence in the fifteenth verse comes due in the eighty-ninth.
Over this last piece of the sentence the lexicographers differ: which is the moment at which they will not be aware — that moment (as they drop him into the well they do not know that the news has been given), or the future moment (as they are being informed they do not know that he is Joseph)? Both readings are transmitted; I do not impose a preference.
The second reading accords with the end of the sūra — in verse 58 it will be said fa-ʿarafahum wa-hum lahu munkirūn. I record this as an indication, not as a preference.
عِشَاء — the darkness of evening; the time after the sun has set. Root ع-ش-و: weakness of the eye's seeing, half-light. From the same root comes aʿshā — night-blind.
I record this as an observation: the naming of the time is not idle. The hour at which faces are hardest to read has been chosen. And this accords with the sense of "the weakening of sight" in the root of the word.
يَبْكُون — a circumstantial clause: in a state of weeping. And that tears are not evidence will be seen in the next verse, together with the shirt.