When is speaking of others' flaws considered disclosure and not backbiting, and when is ridicule permissible or constructive, and what are the conditions for constructive criticism?

Backbiting is to mention a person with what he dislikes. The difference between it and reporting is that reporting is not backbiting if it is intended for treatment, seeking advice for someone who has erred, or guiding them to what is right. However, if the intention is merely to report shortcomings and people's sayings from which no benefit results, then it is impermissible. Mockery is forbidden, as Allah Almighty says: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers." As for advice, it is the foundation of religion, and it must be given in private, not in public.