warn

warn — verb

1. to give information to someone about a dangerous or unpleasant thing that might

1.verbtransitive / intransitiveB1
Definition

to give information to someone about a dangerous or unpleasant thing that might happen, so they can be prepared or avoid it

Example

The tour guide warned the hikers about the sudden storms that appear in the afternoon.

warn + object + about + danger

Rohan's mechanic warned him that the brakes were nearly worn out and unsafe.

warn + object + that-clause

Common collocations
  • warn about the risks
  • warn of danger
  • warn against doing something
  • repeatedly warn
  • fairly warn
  • warn in advance
Synonyms
  • alert

    Stronger sense of urgency; often used for immediate, time-critical dangers

  • caution

    More formal; implies advising careful behaviour rather than informing about danger

  • notify

    Neutral delivery of information without the element of danger or urgency

Antonyms
  • reassure

    To remove someone's worry about a possible danger

Grammar Patterns

warn + object + of/about + danger

warn + object + that-clause

warn + object + to-infinitive

warn + against + gerund

warn + of + noun (intransitive)

Usage Note

Common patterns: 'warn + object + about' for general dangers, 'warn + object + of' is more formal and frequent in official notices. 'Warn + object + that-clause' is the only pattern that allows expressing a full situation. The intransitive 'warn of' does not name a specific person being warned.

Common Mistakes

I warned him the storm.
I warned him about the storm.
'warn' requires a preposition before the danger, not a double-object structure.
The sign warns to drivers about the bridge.
The sign warns drivers about the bridge.
'warn' takes a direct object without 'to'.
She warned that he is late.
She warned that the bridge might collapse.
'warn + that-clause' must describe a future or possible danger, not a current fact.

2. to tell someone firmly that their actions are wrong or dangerous and that they m

2.verbtransitiveB2
Definition

to tell someone firmly that their actions are wrong or dangerous and that they must change them

Example

Bao's mother warned him about the trouble he would cause by skipping classes.

warn + object + about + consequences of behaviour

The shopkeeper warned the teenagers against stealing sweets from the shelf.

warn + object + against + gerund

Common collocations
  • warn about behaviour
  • warn against stealing
  • sternly warn
  • warn twice
  • warn for the last time
Synonyms
  • admonish

    More formal and official; often used in disciplinary contexts

  • rebuke

    Stronger disapproval; implies sharp criticism for something already done

  • caution

    Softer; suggests carefulness rather than strong disapproval

Antonyms
  • encourage

    To give positive support for someone's behaviour

  • praise

    To express approval of someone's actions

Grammar Patterns

warn + object + about + behaviour

warn + object + against + noun/gerund

warn + object + not to-infinitive

Usage Note

Frequently used in school, parenting, and workplace contexts where someone in authority corrects behaviour. 'Warn + object + not to-infinitive' is the most common pattern. Distinguished from sense 1 by focusing on the person's conduct rather than an external danger.

Common Mistakes

She warned him to not smoke.
She warned him not to smoke.
In standard English, 'not to' is preferred over 'to not' after 'warn'.
The teacher warned about cheating the exam.
The teacher warned the students about cheating on the exam.
'warn' needs a direct object (who is being warned) and the correct preposition ('on' before 'exam').
Phrasal verbs

warn off

phrasal verb

to tell someone to stay away from a place, person, or activity, often using threats or authority

The security guard warned the group off after they tried to climb the fence at night.

Adina warned off the stranger who kept knocking on her neighbours' doors asking for money.

Word family

  • warning
    nounB1
  • warning
    adjectiveB2
  • forewarn
    verbC1
  • unwarned
    adjectiveC2
  • warner
    nounC1

Synonym discussion

Sense: verb/1

All four words involve giving information to someone, but they differ in urgency and purpose. 'Warn' and 'alert' both focus on danger: 'alert' suggests immediate, urgent action is needed (e.g., alerting people to a fire), while 'warn' is broader and can cover future risks. 'Caution' is more about advising careful behaviour rather than reporting a specific danger. 'Notify' is neutral — you notify someone of a fact, not necessarily a threat. 'Advise' implies a recommendation or guidance, not a warning of harm. For example, a doctor warns a patient about smoking risks, alerts them to a sudden allergic reaction, cautions them against over-exercising, notifies them of an appointment change, and advises them to rest.

alert ↗caution ↗notify ↗advise ↗

Common collocations

Nouns

Adverbs

Phrases

Prepositions

Etymology

Proto-GermanicOld EnglishMiddle EnglishEnglish

The word 'warn' comes from Old English 'warnian', meaning 'to take heed, beware, or give notice of danger'. It traces back to the Proto-Germanic root '*warnōną', which is related to 'beware' and 'wary'. By the 13th century it had taken on the sense of 'to give advance notice of danger', and by the 16th century it also carried the meaning of counselling or admonishing someone's behaviour.