The phrase "warn us to" is correct and can be used in written English.
It is often used when someone is being cautioned or advised about a potential danger or consequence. Example: The weather forecast warned us to stay indoors due to the impending hurricane.
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Signs warn us to keep our eyes peeled for crocs.
To the Editor: Your article should warn us to look beyond immigration reform.
She is writing to warn us, to assure us that one day or another, we, too, will have to dial 911 when it's already too late, or sit beside the hospital bed as someone slips inexorably away.
Demonstrated single Panmictic stock via phylogenetic analysis can warn us to care about this notable stock in these sites.
For example, if we buy a stock at $30 do we know that the fundamentals we will see in the future will warn us to sell before the stock drops to $20? Remember that without a clearly defined exit point we shouldn't enter the investment in the first place.
Cautious Yanks tried to warn us to stay out of Nicaragua.
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The expedition leader warned us to use seasickness aids before we reached the notorious Drake Passage, where the Pacific, Atlantic and Southern Oceans collide.
He had no details but warned us to be careful.
Bruce Bartlett warns us to beware of budget gimmickry.
Mary Oliver warns us to leave room for the unimaginable.
She warned us to take plenty of water.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com