Is it considered backbiting if I answer someone who asks me about another person’s smoking, knowing the truth of his smoking, given that I don't know the reason for his question? And if I refuse to answer, will the questioner know that this person is a smoker? And is it considered an injustice, incurring a right for others, if I laugh at a mistake committed by someone?

The default ruling regarding backbiting is that it is forbidden. However, it may become obligatory or permissible for a legitimate Shar'i purpose that cannot be achieved except through it. Among the cases in which it is permissible are:

- Warning Muslims against evil and advising them, such as discrediting narrators, witnesses, or those who undertake to issue fatwas or teach Quran while lacking qualifications, or being فاسق (defiantly disobedient), or an innovator.

- When a person openly commits sin (يتجاهر بفسقه) or innovation (بدعته), it is permissible to mention them by what they have openly declared, but nothing else.

As for the inquirer knowing that the person is a smoker due to your silence, this does not harm you as long as the knowledge did not come from your side. As for your laughing at someone's mistake, the reason for the laughter must be understood.