Inspired by “Islamic Rulings: a Guide of Islamic Practices” by His Eminence, Sayyed MuhammadHussein Fadlullah (ra)
What is meant by "enjoining good and forbidding evil" is the enterprise of opposing the person who turns their back to good deeds by counseling them to take to doing deeds and abandoning wrongdoing that they do as a matter of course, by any of the methods laid down by the Sharia. This, however, does not apply to the person who is not in a position to draw a line between what is obligatory and what is impermissible.
What we mean by "good" is every good deed enjoined by the Sharia by way of making it obligatory to uphold or to embark on voluntarily.
What we mean by "evil" is every evil deed the Sharia forbade by making it impermissible to embark on, or at least to keep one's distance from. Should "evil" be of the impermissible type, forbidding it becomes obligatory (wajib); and it bemakrouh (abominable), forbidding it becomes mustahab.
There are several requirements that enable one to enjoin good and forbid evil. In addition to the general requirements of coming of age and reaching a level of maturity, the following are also conditional to enjoin good and forbid evil.
First: knowledge of the right and wrong. One must learn the rulings regarding areas to which a person may be subject to his conduct and relations, so that he can carry out the duty of enjoining good and forbidding evil.
Second: the enjoiner and forbidder must be safe from harm to himself and his honor; their condition includes also no harm to other Muslims or their possession or honor.
It is also conditional that the act of enjoining or forbidding does not require getting into intense difficulty that the person cannot withstand, such that if this leads to him not being able to stay in his house, or to leave a profitable job for a very demanding job. This also applies to enjoining good and forbidding evil that leads others into intense difficulty.