1. warn sb against doing sth 告诫某人不要做某事。如：
The doctor warned the patient against smoking. 医生告诫病人不要抽烟。
The teacher warned the children against playing on the street. 老师告诫孩子们不要在街上玩。
2. warn sb against [about] sb (sth) 告诫某人提防某人(某事) 。如：
He warned us against pickpockets. 他叫我们提防扒手。
The police have warned shopkeepers about the forged banknotes. 警察已通知各店主要留意假钞。
3. warn sb of sth 警告(通知)某人会有某情况。如：
We warned him of the danger. 我们警告过他有危险。
The teacher warned us of the test. 老师通知我们要考试。
【注】warn 一般不能接双宾语。上面句不能说成：We warned him the danger.
4. warn sb that-clause 警告某人……。如：
I have warned him that it is not allowed. 我警告过他这是不允许的。
Police warned drivers that the roads were icy. 警方提醒司机路面已结了冰。
We warned her that it would happen. 我们警告过她会发生这种事。
I warn you that it is dangerous to swim here. 我警告你，在这里游泳是很危险的。
5. warn sb (not) to do sth 告诫某人(不)要做某事。如：
I warned him to be careful. 我告诫他要小心。
I warned him to stay away. 我告诫他别靠近。
He warned me not to go out with her. 他告诫过我不要同她一起出去。
The doctor warned him not to eat oily food after the operation. 医生告诫他手术后不要吃油腻食物。
比较：He warned me not to going out at night. / He warned me against going out at night. 他叫我不要晚上出去。