warn someone about someone or something
to advise someone about the dangers associated with someone or something. Didn't I warn you about the dangers of going there? I warned you about Alice.
warn someone against someone or something
to advise someone against someone, something, or doing something. We warned them all against going to the region at this time. I warned her against Gerald.
warn someone away from someone or something
to advise someone to avoid someone or something. We warned her away from the danger, but she did not heed our warning. Why didn't you warn me away from Roger?
warn someone of something
to advise someone that something bad is likely to happen. I wish you had warned us of what was going to happen. Please warn John of the heavy traffic he may run into.
warn someone off
to advise a person to stay away. We placed a guard outside the door to warn people off until the gas leak could be fixed. The guards warned off everyone in the vicinity.
warn about
v.1. To make someone aware of the actual or potential harm, danger, or evil associated with something: I warned the kids about riding their bikes in the street.
2. To make aware in advance of some actual or potential harm, danger, or evil: The report warned about a possible attack.
warn against
v. To advise someone that something is dangerous or problematic and should be avoided: I warned them against driving without seatbelts. The doctor warns against smoking.
warn away
v. To notify someone to go or stay away: The guide warned the tourists away from the edge of the cliff. The sign warned away trespassers.
warn of
v. To make someone aware in advance of some actual or potential harm, danger, or evil: The doctor warned them of the flu epidemic. The employees were warned of the company's impending bankruptcy.
warn off
v. To notify someone to go or stay away: The sheriff warned them off the private property.