"be used to warn" is a grammatically correct phrase that can be used in written English.
It means that something is commonly or regularly used for the purpose of warning someone about something. Example: "The loud siren is often used to warn residents of an impending tornado."
Pakistan's nuclear weapons can be used to warn India not to advance on Pakistani territory.
This could be used to warn cyclists of the most dangerous routes and help prioritise spending on new bike lanes.
Stridulation in larvae is independent of sex and may be used to warn neighbouring larvae to avoid getting in each other's way.
It could be used to warn troops about a threatening vehicle or to circle a compound that a drone should attack.
Google will also continually download a list of phishing sites and list of malware sites to your computer, which will be used to warn you when you visit them.
Text messages can also be used to warn farmers about potential problems with their crops and collect data on farming trends.
Books which have been named by students as potentially requiring "trigger warnings" – the term is used to warn readers of possibly distressing material to follow – include Things Fall Apart by Chinua Achebe, The Great Gatsby by F Scott Fitzgerald, Mrs Dalloway by Virginia Woolf, and The Merchant of Venice.
Television adverts, social media posts, letters and radio campaigns have been used to warn claimants about fraudulently claiming benefits.
Since then, they have been used to warn residents to lock their doors when deputies were searching for criminals, but mainly to evacuate people from smaller fires.
There are plans for sturdier chairs and planters are expected to replace the orange plastic barrels now being used to warn cars away.
Each of you had the gun at one point or another and it was used to warn off any male member of the public who looked as though he might intervene.
