﴿ فَلَمَّا ذَهَبُوا۟ بِهِۦ وَأَجْمَعُوٓا۟ أَن يَجْعَلُوهُ فِى غَيَٰبَتِ ٱلْجُبِّ ۚ وَأَوْحَيْنَآ إِلَيْهِ لَتُنَبِّئَنَّهُم بِأَمْرِهِمْ هَٰذَا وَهُمْ لَا يَشْعُرُونَ ﴾

“And so, when they went away with him, they decided to cast him into the dark depths of the well. And We revealed [this] unto him: "Thou wilt yet remind them of this their deed at a time when they shall not perceive [who thou art]!"”

So when they went off with him and agreed resolved to put him into the bottom of the well the response of lammā ‘when’ has been omitted in other words understand it as being fa‘alū dhālika ‘they did just that’ by taking off his shirt — after having beaten him humiliated him and desiring to kill him — and lowering him in a bucket into the well. When he reached half-way down the well they flung him from the bucket so that he would fall and die but he fell into the water and took refuge near a rock. They then called to him and he responded thinking that they had decided to have mercy on him. They then wanted to smash his head with a rock but Judah prevented them. And We revealed to him in the well through true revelation wahy haqīqa — he was 17 years of age or just under — in order to reassure his heart ‘Truly you shall inform them after this day of this affair this deed of theirs when they are unaware’ of who you are wa-hum lā yash‘urūna is a circumstantial qualifier referring to the action of ‘informing’.