Will Allah conceal the sins of one who masturbated while imagining women he knows, then repented, or will He expose him on the Day of Resurrection? And what is the way to attain Allah's forgiveness and concealment?

Masturbation is a bad practice that should be avoided. Repentance has its conditions, and you must conceal your sin, for the Prophet, peace and blessings be upon him, said: "All of my Ummah are pardoned except for those who publicize their sins." Part of concealing your sin is that if one commits an impurity, he should cover himself with Allah's cover. And whoever Allah covers in this world, He will forgive him in the Hereafter. Persist in seeking refuge from Satan and hasten to marry if you are able; otherwise, you must fast.