例句：
1. He spends his spare time visiting schools to warn pupils off drugs.
他用自己的业余时间访问学校，告诫小学生远离毒品。
2. Signs across the entrances warn all visitors to stub out their cigarettes.
入口处的告示牌提醒游客们熄灭香烟。
3.I must warn you against raising your hopes.
我必须警告你不要期望过高。