to make someone conscious of a possible problem or danger so that they will not be hurtwarn someone to do something: Police are warning all women in the area to take extra care when going out alone.
warn against: Recent studies warn against drinking too much caffeine.
warn someone about something: Travel agents are not warning tourists about the dangers of crime in holiday resorts.
warn of: Scientists warned of the threat to beaches and rivers from pollution.
warn that: The report warns that consumers could end up paying higher prices.