warn的用法是：warn只用作及物动词,可接名词、代词。
warn SB (not) to do 警告、劝诫、劝说某人（不）要做某事
这个用法可以转化为被动语态：SB be warned (not) to do ,
The doctor warned the patient not to eat oily food after the operation.
The patient was warned not to eat oily food after the operation by the doctor.
医生警告过病人在手术后不要吃油腻食物。
I warned you not to buy that old car.
我劝过你不要买那辆旧汽车。
They wre warned not to climb the mountain in such bad weather.
有人劝诫他们不要在这么坏的天气去登那座山。
warn的其他用法：
1、warn of 警告……关于（危险等）；预告说……；通知说
Police have warned of possible delays.
警方已经通知可能会廷期。
Please give us advance warning of any changes.
如有变动，请事先通知我们。
It seems only right to warn you of the risk.
似乎应该警告你有风险。
2、warn sb off (sth) （尤指以威胁的方式）叫…离开，告诫…不要靠近；
The farmer warned us off his land when we tried to camp there.
我们想在那里露营时，农场主警告我们不得靠近他的土地。
3、warn sb off doing劝…不要做；建议…停止做
We were warned off buying the house.
有人劝我们不要购买这所房子。