Warn
Incorrect : She was warned with/about the danger.
Correct : She was warned of the danger.
Incorrect : I warned him of his disobedience.
Correct : I warned him against his disobedience.
Note: We warn a person of a danger, but against a fault.
Warn
Incorrect : She was warned with/about the danger.
Correct : She was warned of the danger.
Incorrect : I warned him of his disobedience.
Correct : I warned him against his disobedience.
Note: We warn a person of a danger, but against a fault.