From Longman Dictionary of Contemporary Englishwarnwarn /wɔːn $ wɔːrn/ ●●● S3 W2 verb [intransitive, transitive] 1 WARNto tell someone that something bad or dangerous may happen, so that they can avoid it or prevent it ‘Be careful, the rocks are slippery, ’ Alex warned.warn somebody about something Travellers to Africa are being warned about the danger of HIV infection.warn (somebody) of something Salmon farmers are warning of the severe crisis facing the industry.warn somebody (not) to do something I warned you not to walk home alone. Motorists are being warned to avoid the centre of London this weekend.warn somebody (that) We warned them that there was a bull in the field.2 WARNto tell someone about something before it happens so that they are not worried or surprised by itwarn somebody (that) Warn her you’re going to be back late.RegisterIn everyday English, people often use the expression let someone know rather than warn someone:Let me know if you’re not going to finish on time.GRAMMAR: Patterns with warn• You warn someone that something will happen (=you say it will definitely happen): I must warn you that the treatment will have side effects.• In the past, you say that someone warned that something would happen (=they said that it would definitely happen): Campaigners warned that the airport would harm the environment.• You warn that something might happen or could happen: He warned that there might be another war.• Warn is also often used in the passive. You say that someone was warned about something: We were warned about the dangers of global warming.THESAURUSwarn to tell someone about something bad or dangerous that might happen, so that they can avoid it or prevent itI warned you about sitting out in the sun too long.We were warned that there could be delays on the motorway, so we took another route.give somebody a warning to tell someone that if they continue to behave in an unsatisfactory way, they will be punishedHe’s already been given several warnings about handing in his essays late.The US gave a warning that if the hostages were not released, it would be forced to take military action.alert to officially or publicly warn people of possible danger so that they can prevent it or be ready to deal with ita campaign to alert people to the dangers of smokingAn anonymous caller alerted the police that a bomb was due to go off.tip somebody off informal to secretly warn someone about something that is going to happen – used especially about warning the police about a crimeThe police found the drugs after being tipped off by local residents.Informants tipped the FBI off.caution formal to warn someone to do or not to do something in order to avoid a dangerous or bad resultPeople are being cautioned against using credit cards abroad, in case of fraud.Health officials have cautioned the public to wash fruit thoroughly before eating it.forewarn /fɔːˈwɔːn $ fɔːrˈwɔːrn/ [usually passive] formal to warn someone about something that is going to happen, so that they are expecting it or ready for itWe had been forewarned that the roads weren’t very good. → warn (somebody) against something → warn somebody ↔ away → warn off → See Verb table
Examples from the Corpus
warn• Tourists are warned against going to remoter regions.• But they should at least be warned, and wary.• Police are warning drivers not to go out on the roads unless their journey is really necessary.• We tried to warn her, but she refused to listen.• He would be the one Rostov had warned him to be wary of.• Something warned Lucy that she must stop him.• I was going for a swim, until the people in the hotel warned me about the jellyfish.• When I married my first Guenever you warned me that the marriage might be unwholesome for me.• Weather forecasters warned of possible flooding in coastal regions.• Dark clouds warned of the approaching storm.• A senior official in the State Statistical Bureau warned that demand had to be controlled to avoid runaway inflation.• Cooley warns that one should go slowly and not slip at the last rung of the ladder.• The local people were warned that the volcano might erupt at any time.• The bleached bones of cattle warn the traveler how hot it can be in Death Valley.• We were warned there were going to be delays on the motorway, so we came back by a different route.• The consequences could be serious - I just wanted to warn you.• Just warn your friend Martin to be extra careful, because everything he does is under close scrutiny.warn somebody (that)• Be warned that the photos contain graphic violent images.Origin warn Old English warnian 