A person is obligated to rebuke a person from sinning
It is a mitzvah for a person who sees that his fellow Jew has sinned or is following an improper path [to attempt] to correct his behavior and to inform him that he is causing himself a loss by his evil deeds as [Leviticus 19:17] states: "You shall surely admonish your colleague."
A person who rebukes a colleague - whether because of a [wrong committed] against him or because of a matter between his colleague and God - should rebuke him privately. He should speak to him patiently and gently, informing him that he is only making these statements for his colleague's own welfare, to allow him to merit the life of the world to come.
If he accepts [the rebuke], it is good; if not, he should rebuke him a second and third time. Indeed, one is obligated to rebuke a colleague who does wrong until the latter strikes him and tells him: "I will not listen."
Whoever has the possibility of rebuking [sinners] and fails to do so is considered responsible for that sin, for he had the opportunity to rebuke the [sinners].
Is this only when violating a negative commandment?
If someone is violating a positive commandment must/should/can he be rebuked (in private...)?
For what commandments should/must/can rebuke be given?
Is the commandment to say the Shma the same as to put on tzitzis in regards to this?