Well, "to warn" has the aura of "make sure that you don't"; if you are warned, you're expected not to do something... " I don't feel that "warn to do something" sounds right... "
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Wistiti2I don't feel that "warn to do something" sounds right... You'd rather say "our teacher told us to ...", or "advised us to..."I agree that it does sound a tad strange, but I know that I've heard it, and perhaps even used it. So it isn't extremely hard on my ears.